I'm looking at an 87 boat with 425's in it...
Does anybody know anything about these, good or bad?

Here's what I have found so far:

Merc. 425's were available 1990-1993. 454 V8

The Merc. 420's were available in 1987. 454 V8

Any idea how much horsepower these things made?

Do a search in the Tech section for the 400 cyclone and 420 & 425 hp engines. There have been a few threads on them.
If I remember all the comments correctly, good engine, good parts in it. I'm thinking there were some neagative comments on the cam and/or ignition though.
Gary

My understanding is that the 420 and the 425 were the same engine. Badge engineering. I had a pair of 420's in a 30 Scarab. Strong engine, quality parts. Some problems with exaust reversion or leaks getting water in engines. Merc went to Gils soon after .

This is a good motor. Any time you can get your hands on a mark IV block, get it. As far as 420 and 425 being the same, thats correct. Even the 400 cyclone is the same motor, mercury just added Gill exaust and called it a 420. Go to www.Bammarine and go to the mercruiser parts listing. You can compare parts used in these motors all the way back in the 70s.

Thanks guys, sounds like these are a stout engine, thats good news. ( thats more HP than a 502 Mag)
This boat is looking like it may be a keeper