When I've built my Seal Enclosure, I've used Rocks to build a Ramp because it was too steep for them. I assume that's one of the best Methods. If this doesn't work, try to find out if it's because of the giant Hitbox of the Animal.
For measuring the Depth I've used a Piece of Wall. It really needs to be 4m deep. Even if it's only a few Centimeters too shallow, the Animals will be way too unhappy (which was really annoying when I've built it because I was so sure it is a Bug). Be also aware that the Underwater Feeder needs to be a little bit deeper than 4m to work
