That is an inline strainer - it's supposed to stop crap in the water from clogging up your transmission cooler. The bottom unscrews, the screen comes out, and you can empty whatever organic material has accumulated.
There's a tag on the transmission that identifies it - I think our boats have a 630v. Look at the backside of the transmission under the center rear seat (pull out the insert under the seat).
Checking the transmission cooler is a bit difficult - it's just a heat exchanger, so if you can pressurize one side or the other you can check for a leak.
1998 Maristar 200VRS