Haven't played MoF. No 3DS, sadly.
And you can swim in Aria/Dawn and Symphony, too, but holes filled with water seem to be the Belmonts' number one enemy.
I actually have a theory about that. It is based on how the whip was completed in LoI. To turn the Whip of Alchemy into the Vampire Killer, Leon had to go through a ritual in which he needed someone tainted by a vampire (Sara). My theory is that because of that ritual not only was the whip completed, but Leon was also granted the power to compete with vampires on near equal footing by giving some vampiric power which is passed on to his descendants. Don't believe me? How else was Trevor able to survive being stabbed by Issac in CoD. Yes, he nearly died, but I heard something about him getting up and leaving the following day. Any normal human would have died. And, if by some miracle they survived, they would be bedridden for a while. Hell, in real life, despite our advanced health care, people who are stabbed in that area are still in intensive care for a few days. Hence, why I think the Belmonts have some vampiric power. Well, at least in terms of surviving wounds and healing. And, if this is the case, I believe the price for all of this is not being able to swim. After all, in most of the vampire lore I know, vampires have issues with water. Thus, if the Belmonts indeed have even a small amount of vampiric power, this could account for the lack of swimming.
Don't kill me, but this is something that popped into my head when I heard that Trevor walked away after being stabbed like that in CoD. Then again, it's only something I heard from someone so I can't say if that's the case. If it isn't, then maybe my theory is wrong. But still, the fact that the Belmonts can't seem to swim and that ritual makes wonder....