tl;dr - yes, you can make a surface ship submersible, but it's done at the expense of typically increased construction cost and reduced dynamic performance when operating in either domain (i.e. surface or submerged).
I'm not sure what you mean by 'circlular', but I think it's safe to assume you mean a circular hull cross section vice a round circular deck (like a floating helo pad or SpaceX launch barge).
First, there have been some examples of surface ships that transition to submarines (e.g. 1). China is apparently working on a large Surface ship/submarine for their Navy.
Surface and submersible vessels have very different hydrodynamic and hydrostatic properties and forces to contend with, which drive them to have different shapes. I actually did work on a recent project that explored this topic a bit. Generally, for any vessel, the shape depends on what you want the ship to do and hull form/type selection is very much a science-backed art form of Naval Architects.
You can certainly build a ship to fully submerge, the question is, particularly at larger scales, is it worth the added cost, risk, and hydrodynamic compromise - or are you better off just building two different vessels that are specifically designed to operate in one domain (I.e. on the surface or submerged). That answer is highly dependant on what you intend to use the vessel for. It may be worth it for your application, but in general the answer has been that it's more cost-effective to just optimize vessel design for one operational domain rather that two.
Regarding hydrodynamic trade, while both ships and subs strive to minimize total hydro resistance (drag), hydro resistance has subcomponents that affect vessels differently when operating on the surface versus submerged. Surface ship shapes strive to be hydrodynamically 'optimized' to minimize form, skin, wave (critically), and even air resistance, whereas submarines aren't generally subject to wave action or wind and thus typically focus on minimizing the dominant form and skin drag forces. Surface ships are generally more power efficient than submarines at equal displacement and speed through water.
If you want to make a ship submerge, you also need to add significant structure to withstand hydrostatic forces at depth. Submarines are inherently structurally sound enough to operate at the surface, but function hydrodynamically poor at the surface because of their shape.
Surface ships and submarine shapes primarily differ in structure due to the significantly different pressure forces they need to contend with. Submarines have a circular cross section because, next to a sphere, cylindrical hulls are the most structurally stable pressure hulls. Surface ships can actually have relatively structurally 'weak' main decks, operating almost like floating buckets that are mostly designed to stay right-side up. The weaker main deck makes the ships cheaper to build, but probably more critically, allows easy access to whatever payloads the vessel is carrying.
1. https://www.thedrive.com/the-war-zone/9726/the-usmc-is-inter...