It is absolutely feasible. I presume you are referring to plastic kits. The main thing to be aware of is that when you cut away the bottom of a hull, you are increasing the likelihood of introducing slight A-symmetries to the hull, as various internal tensions are either released or exacerbated. The larger the scale, the more pronounced you can expect this issue to be.
For example, I cut away the problematic lower hull of Heller’s Soleil Royal. My mistake was in cutting each hull half separately, before gluing it together. This immediately created several problems that were challenging to overcome.
First of all, it was tricky to flatten that cut line without pulling the stem out of plumb. I happened to be widening the hull, at the stem, so there was a natural opportunity to correct that problem. It was hella-tedious, though, and it would have been so much simpler to have glued the hull together, first, so that I could get a consistent cut line all the way around the hull. Flattening all of this at once would have been much simpler.
Secondly, despite taking great pains to place the hull halves, clamped together at the stem, upon a base sheet of 1/16” styrene, so that I wasn’t introducing any additional tensions - I didn’t realize that the port and starboard sides were no longer following the same hull curvature: the starboard side had become a little flatter, now, and this only became apparent to me when it came time to make the new upper decks from scratch. As I was lining-out the planking strakes, I could see that the layout would be close, but not symmetrical. I’ve gotten to be very good at coping mismatched parts together, but I could have made this much easier, in the first place.
So, my advice to you - before you cut anything - would be to glue your hull halves together first. Cut ample internal bracing for the hull’s interior, so that the breadth and curvature of the hull remain consistent during and after the cutting; this bracing could take the form of full or partial bulkheads, which will add quite a lot of structural rigidity to the model. In larger scales, I would say this is essential. In smaller scales, you could probably get away with simple athwart-ships rod or stick bracing.
If you are building steel navy ships, it would probably also be advisable to glue-in the main deck, in order to preserve the hull shape.
For the cutting, access to a well-tuned bandsaw makes the quickest and easiest work of this. I would build a simple balsa carriage to hold the model’s waterline perpendicular to the bandsaw table.
Afterwards, you can tape 100 grit paper to either a glass table top or a metal circular saw table, or to a board that has been jointed dead-flat, to grind the rough edge down to a uniform waterline.
If you don’t have access to a bandsaw, you can achieve very good results with a Japanese-style pull-saw. Just mark off your waterline and use brightly contrasting painters’ tape along the waterline so that you can more clearly gauge your progress. Be sure to leave yourself a heavy 1/32”, away from the line, for final fairing. In larger scales, I would leave maybe a 1/16”.
Lastly, be sure to make some allowance for the hull to sit in whatever sort of diorama sea you have in-mind.
