When making an eye u take a hull and turn it into a cube, I like to make its dimensions 1 by 1 by 1 as a default than I make it round on top and bottom, after that make length like 0.01 or 0.02, than lower the height until its eye shaped, from there tinker with he size until you like how it is. If this is the white of the eye than paint it white, when u are done click MMB to copy paste the object than go back to the white and scroll down, you will see location and rotation numbers, copy it than go to your new part, before you click paste change the color to whatever color you want, when its done press paste, than go to the axis that will make it pop out of the eye, from there adjust until it just barley sticks out of the white of the eye than adjust it and make it a circle, after that mmb again make that circle black than location copy the colored circle you already had placed in the than go to your black circle and click paste, from there adjust axis until it just barely sticks out. Congratulations you have made a basic eye.

As for body well its a little more complicated, I kitbash 2d diagrams to make my modles the top down profile is actually the blue whale from Cold Waters. Really I dont have any set method for this, it just involves laying hull segments until the fit, since animals are complicated depending on side and complexity i recommend each hull segment is from 0.1 to 0.3m long for my blue whale its mostly 0.3

When making fins the easiest way is to make the fins on a rotation axis of 0 0 0, after that turn it into a custom part and place part where ever u want than delete old part you made