I think another reason why you couldn't swim in GTA3,VC, & LCS is that in even when there's swimming in VCS and SA, it wasn't easily to get back on land, and docks and etc were hard to find, especially on the right middle shoreline of VCS and along some rivers north of Los Santos in SA, so IMO, swimming REALLY sucked. I hate being stranded in the water, and in VCS you couldn't commit suicide in anyway in water to get transported to the nearest hospital.
Besides, when cops were in the water, why couldn't they swim?