Name a game where the swimming mechanic is good. Mario? Maybe when he has the shell in the 3D games. Perhaps the swimming in the original Super Mario Bros. was not bad but the underwater stages were at least the most annoying parts, right? I do not mean that the swimming suffices or is okay, I mean genuinely good to the point that it is enjoyable. I honestly cannot think of one game that I have played.
The best was perhaps the Mega Man games in that there was no swimming and instead the Blue Bomber's jump physics got wonky instead. Of course Mega Man 8 seems to want to buck that trend by introducing really, really weird swimming physics. You jump once and then the next jumps give you little upward blurps of movement that feel just plain old bad.
I get that swimming is not easy. I know that I am less ""good at swimming"" and more ""have not died in water yet"" and I do grant that without trying we will not find swimming mechanics that are actually enjoyable. I just know I have yet to see any.