Trimming buoyancy instead of driving or walking
On land, moving from one place to another and staying upright are largely separate problems handled by wheels, tracks, or legs pushing against a solid surface. Underwater, a vehicle is suspended in a fluid that pushes back on it from every direction, and the vertical dimension in particular is governed by buoyancy rather than by anything resembling wheels or legs. Buoyancy is the upward force the water exerts on a submerged object, and whether a vehicle floats, sinks, or hovers in place depends entirely on how that force compares to the vehicle's weight. A vehicle that's neutrally buoyant, meaning its buoyant force exactly balances its weight, will hover in place without needing to actively fight gravity, which is a fundamentally different resting state than anything a legged or wheeled robot has, since those need continuous support from the ground or continuous motor effort to stay off it.
Achieving and maintaining that balance is trickier than it sounds, because a vehicle's buoyancy isn't fixed. Water density changes with depth, temperature, and salinity, and a vehicle's own volume can compress slightly under pressure at depth, both of which shift the buoyancy balance as the vehicle moves through the water column. A vehicle trimmed to be neutrally buoyant near the surface may end up slightly negative or positive buoyant at depth, so buoyancy control isn't a one-time setup step, it's an ongoing adjustment problem that has to be actively managed throughout a mission, especially for vehicles operating across a wide depth range.