So with regards to Baseball and...
Scuba Club = Yes, since they only can do weekend dives and trips. And you don't have to go on every dive/trip to be apart of the scuba club.
Infantry Skills Team = No, they're a full time team and just as selective as a team as the Combat Arms Team.
If you're looking for a ECA/club that involves shooting and basic infantry tactics that would also work with Baseball, I would look at the Semper Fi Society. They occasionally do shoots, fly people down to Camp Lejeune on Ospreys for a weekend of Marine Corps activities every semester, and have monthly combat speakers. Even if you aren't currently looking at Marines as your service assignment, it would worth a look.
Hope this helps.