If it is 30M or more, then it should be fine for swimming. I probably wouldn’t want to take a watch into a shower or a hot tub but that is purely personal preference.

I think “splash resistant” is generally 3M? I am probably not taking it deeper than a meter or two, and swimming motion probably adds another meter, but why take chances? Unless you only have one watch, then you should have at least one watch with a much higher water resistance that is more suitable for swimming.

I would be most worried about making sure the crown is properly screwed down or pushed in (as the case may be). That is the weakest point of the watch where water is most likely to get in.

3-5 years is pretty frequent as far as servicing intervals go.

At a certain point, it’s cheaper to just replace the whole movement than it is to get it serviced. Even if it is most of the price of a new watch.