The moonpool needs you to float up into your base, so you need clearance underneath it. I usually build a base on the edge of the shallows for this.
This is my base atm ( I also shouldn’t have taken the pics at night I now realise)
The square one is the Moonpool and the circle beside it is 2 floors with storage and fabricators and mod stations etc.
The 2 cylinders on the far side are giant 2 story fish tank, water purifiers, indoor growbeds and a bio reactor. Since I took these shots I have put another room beside these with a nuclear reactor. It is no way the best use of anything but I like it. The exterior beds have mushrooms and kelp stuff for when I need them.
As for getting below 300, there is a mod for the seamoth for that or you can look at the spoiler
Go to the Aurora crash site, and you can go in and find a storage module and a depth module to upgrade your seamoth.