Bellows, there are three of them the largest top one is the drive shaft bellows and seals off the universals from the water. Smaller bottom bellows is the exhaust, if this leaks water cannot enter the bilge. Then there is a small tapered shift cable bellows.
If anyone is torn it's time to replace all three. Since you are getting water in the boat replace the gimbal bearing as well.
Go to Documents of this site to get the mercruiser manual for the drive and read up.
1997 2400 SCR
5.7 Vortec / Bravo 2