+1 This would increase immersion.
...ISWYDT (even if, technically, it does the reverse
)
Serious answer: needs to properly take account of buoyancy.
Then you can:
...enable walking of along any bottom[1] for
any creature able to survive the liquid and being able to respire/not needing to/not having to for long enough. That applies if the creature is dense enough (bronze colossi?) to actually sink, and then be strong enough to make progress through the liquid/against any theoretical current.
...allow swimming across any liquid that they can at least traverse safely take breaths out of, depending upon Swimming skill for how capable/willing/competent they are to do so, for anything even marginally less dense such that they can poke their nose/mouth/trunk/spiracle(s) above the surface at need. Optionally submerge, if not too underdense, to temporarily path under floored-over sections or to take lower-Z through-ways, with the expectation of a receiving chamber within breath-range.
...for the
very buoyant, swimming is impossible and 'surface-walking' (a peculiar skill, unrelated to swimming) is the only chance.
And the dimensions (and favoured attitude!) of the creature dictate when the
lack of buoyancy means that the motion is by bottom-walking even while not at all fully submerged in a given depth, which results in traversal by 'wading'.
Of course, this totally changes much of the existing gameplay physics one expects. Needs to take into account the weight of clothing/armour/baggage and items held and/or carried, as well as innate creature qualities and especiallg personal body style (floaty-fatty or sinky-skin-and-bony) when on the very edge of multiple modals. Expect more resistance than merely the necessarily calculated fluid-drag coefficient, but instead from the die-hard player-base who would appreciate it being an INIT-disablable aspect, if only for the complications it adds to the pathfinding algorithm...
[1] Arbitrary depth, give or take cumulative pressure effects, so long as there are sufficient ramps/steps/climbable-Z-level-transitions to bring them back to the surface when they wish, with however many stacked 7/7 liquid-cells beneath the top one.