Actually making them vermin isn't going to change their stats much except make them a little weaker. Swim speeds aren't given in squares because of vertical movement (same for burrow, climb, and fly speeds).
I also own a copy of KQ 6, so I went to the source. It actually looks like there is a mistake in the original stat block. The modifier on the crab is +2 racial modifier on Swim, but because it has a Swim speed the modifier should be +8. The original author forgot about this.
Type change to Vermin. , HD and BAB aren't changed and skills and feat stay the same as long as it still has Int 1 and you don't make them mindless. Vermin don't have good Reflex saves so reduce it by 2 and change low-light vision to darkvision. That's about it.A creature with a swim speed can move through water at its swim speed without making Swim checks. It has a +8 racial bonus on any Swim check to perform some special action or avoid a hazard. The creature can always can choose to take 10 on a Swim check, even if distracted or endangered. The creature can use the run action while swimming, provided it swims in a straight line.