Heyo! SpinoDude here. 
I am seriously in love with this game- loved doing a play through and have been adoring it in my spare time as well.
The lagoons and marine reptiles are beautiful to look at, but leave a lot to be desired compared to basically everything else. Here are some ideas I had that could help improve them.
Marine reptiles could benefit from more in-depth environmental requirements just like the dinosaurs and pterosaurs. Right now, they just need open space, and a fish or shark feeder to be happy.
"Aqua"forming
Regarding open space, it's annoying that the only way to provide extra space for the marine reptiles is to expand the lagoon- which sometimes isn't an option depending on your park. I think if we could utilize the "raise" and "lower" terrain tools to increase or decrease the depth of the lagoon, it would help drastically for adjusting the needed amount of space for your marine reptiles. You could have an expansive, shallow-water lagoon for a group of Plesiosaurus- or a super deep, intimidating abyss for your mighty Mosasaurus.
Even if we couldn't shape the lagoon floor's terrain to our liking, the ability to control depth would be a great management addition for the aquatics and would also look great visually.
Aquatic Tab: New Terrain Tools
I think we could use an aquatic section under the landscaping tab- and these new terrain tools could provide new unique needs for each marine reptile, giving us a better chance to encourage certain territories to be formed in certain areas of our lagoons.
Aquatic Vegetation Brushes
Below are some ideas for some unique underwater brushes to spice up your lagoons and make your marine reptiles happy.
Kelp Brush: Painting with this brush creates a dense kelp forest, which smaller aquatics like the Ichthyosaurus hang around to take refuge from larger predators.
Coral Brush: Painting with this brush creates a colorful coral reef environment, which draws in small fish from the fish feeders. Some social fish eaters like the Plesiosaurus hang around to catch them.
Seagrass Brush: Painting with this brush creates a covering of decorative seagrass, which some aquatics might find to be a pleasant addition to their environment.
I think these additions alone would add so much diversity to the currently barren underwater environments in-game.
Aquatic Rocks
The ability to place rocks underwater is a no brainer in my opinion, and I think a selection of new coral rocks and barnacle rocks would be appropriate.
Aquatic Painting
The ability to paint dirt, sand, and rock textures underwater would be great, and would help add to marine reptile needs. The addition of some unique aquatic brushes would be welcome as well- perhaps a seashell brush, painting some clams etc onto the lagoon floor.
Floor Feeders
If the bottom of the lagoon is able to be customized, I think the ability to place fish and shark feeders on the lagoon floor would be appropriate. If placed on the bottom, a metal door would open similarly to land feeders- releasing live prey to swim freely in the lagoon. If you want to place the feeders on the side of the lagoon like before, you simple hover your mouse over the lagoon edge and they will snap to an attachment point in traditional fashion.
In Conclusion
While I would love to see all of these features implemented, any of them would be incredibly welcomed. I still absolutely adore this game and I have complete faith in you (Frontier) for making it even better. Your first free update was a great start- and I have some more ideas on other topics I will be posting about coming up. If you made it this far, thank you for reading!

I am seriously in love with this game- loved doing a play through and have been adoring it in my spare time as well.
The lagoons and marine reptiles are beautiful to look at, but leave a lot to be desired compared to basically everything else. Here are some ideas I had that could help improve them.
Marine reptiles could benefit from more in-depth environmental requirements just like the dinosaurs and pterosaurs. Right now, they just need open space, and a fish or shark feeder to be happy.
"Aqua"forming

Regarding open space, it's annoying that the only way to provide extra space for the marine reptiles is to expand the lagoon- which sometimes isn't an option depending on your park. I think if we could utilize the "raise" and "lower" terrain tools to increase or decrease the depth of the lagoon, it would help drastically for adjusting the needed amount of space for your marine reptiles. You could have an expansive, shallow-water lagoon for a group of Plesiosaurus- or a super deep, intimidating abyss for your mighty Mosasaurus.
Even if we couldn't shape the lagoon floor's terrain to our liking, the ability to control depth would be a great management addition for the aquatics and would also look great visually.
Aquatic Tab: New Terrain Tools

I think we could use an aquatic section under the landscaping tab- and these new terrain tools could provide new unique needs for each marine reptile, giving us a better chance to encourage certain territories to be formed in certain areas of our lagoons.
Aquatic Vegetation Brushes

Below are some ideas for some unique underwater brushes to spice up your lagoons and make your marine reptiles happy.
Kelp Brush: Painting with this brush creates a dense kelp forest, which smaller aquatics like the Ichthyosaurus hang around to take refuge from larger predators.
Coral Brush: Painting with this brush creates a colorful coral reef environment, which draws in small fish from the fish feeders. Some social fish eaters like the Plesiosaurus hang around to catch them.
Seagrass Brush: Painting with this brush creates a covering of decorative seagrass, which some aquatics might find to be a pleasant addition to their environment.
I think these additions alone would add so much diversity to the currently barren underwater environments in-game.
Aquatic Rocks

The ability to place rocks underwater is a no brainer in my opinion, and I think a selection of new coral rocks and barnacle rocks would be appropriate.
Aquatic Painting

The ability to paint dirt, sand, and rock textures underwater would be great, and would help add to marine reptile needs. The addition of some unique aquatic brushes would be welcome as well- perhaps a seashell brush, painting some clams etc onto the lagoon floor.
Floor Feeders

If the bottom of the lagoon is able to be customized, I think the ability to place fish and shark feeders on the lagoon floor would be appropriate. If placed on the bottom, a metal door would open similarly to land feeders- releasing live prey to swim freely in the lagoon. If you want to place the feeders on the side of the lagoon like before, you simple hover your mouse over the lagoon edge and they will snap to an attachment point in traditional fashion.
In Conclusion

While I would love to see all of these features implemented, any of them would be incredibly welcomed. I still absolutely adore this game and I have complete faith in you (Frontier) for making it even better. Your first free update was a great start- and I have some more ideas on other topics I will be posting about coming up. If you made it this far, thank you for reading!