Be aware only from very first 32 c.u. OMC 25 (1977-1978), because the gearcase is exactly the same of 22 c.u. 25, so the clutch dog and water pump are too small to resist to the "real" full power.
In 1979 OMC did some basical improvement ( new "three teeth" clutch dog and gears, splined propshaft, plastic water pump, longer tiller handle and improved driveshaft) so the "521" became invulnerable … 😆

In salt water the inner (and the inner only) exhaust duct can perforate near the PW, and the motor became noisier, but isn’t a veritable problem.

Here You can find an exhaustive list of the possible problems … in italian only, sorry…

http://www.oldribs.superweb.ws/NOFRAME/ … %20521.htm