Hi,
It all depends what you are looking for in a submarine, Personally I would not attempt to modify a Revell Gato as a diving submarine IF this is your first venture into submarines. ( there is a lot of complex modifications to do )
If it made as a surface runner only then its not a submarine is it ? only a surface boat that looks like one . Plus it will have a low freeboard and will be easily swamped by
" others " unless it is well high up in the water then it will roll about on the surface as it will not be deep enough in the water to give it the stability it needs ( all subs are larger beneath the water than on top of it ) for this reason etc.
The cheepest way to get a sub that dives is to purchase a ROBBE Sea Wolf, easy build and a dynamic diver IE it goes under with forward motion only and will surface when power is cut or slowed.
It is one of the cheepest ways to get a proper submarine. and not complex at all.
It will teach you the fundamentals of Model submarines and how they work and you WILL learn a lot about the forces that work and the resulting effects they have on a model.
With the addition of a levelling device you will with a little practice be able to run the sub at a constant periscope depth which is quite satisfying.
If you take to being a "subbie" then the experience you have gained with this model will assist you greatly in you next model which no doubt will be a static dive IE it goes under by taking water on board and will dive statically.
Hope this is of some use to you IT IS addictive though.
Good hunting.