What is the advantage/disadvantage of running crossovers verses a circulation pump. I wanted to clean up my engine's a little and crossovers look so much cleaner than the pumps do.

Theres a lot of debating on this subject. Some feel that crossovers lead to high water pressure, issues when running T-Stats, running to cold, etc.

My opinion, is properly rigged, crossovers work fine, and look much cleaner.

Water PSI relief valves are a good idea in any system. If you need to run thermostats, you need a crossover with a bypass.

I use a crossover with bypass hose and thermostat. You will need to drill a couple of small holes in the thermostat to relieve the steam.

Cross-overs are great. They do certainly clean up the front of the engine, you lose the weight of circ pump, and some people believe they free up a negligible amount of horsepower.

Done right, you should have no issues. I recommend one with a by-pass, a drilled-thermostat to allow full-time water flow through the block even when cold, and a pressure-relief valve to prevent any over pressure problems.

You should have a water pressure gauge also.

Its not cheap and in my opion not worth it. The boat I have now has all the above on it. It came with water pressure issues and I had to install the relief valves and water dumps.
Price a Merc relief valve and that will probably determine if you want to do this or not.

Yes, you save some weight, gain a few hp and get a clean front. But is it worth it? Probably not. Just bought a boat running crossovers without thermostats. Engines are running to cold. Now installing 140 thermostats, drilled, and a 3/4" bypass between crossover and tstathousing.

I might also use a pressure relief valve, this because people says so. But since the relief valve doesn't allow greater flow (area) than the bypass, I don't really understand the reason. I mean - i dont think the exhaust makes that much resistance. Or am I wrong?

Put a water pressure gauge on it. You will be surprised.

Mine would peg 40lbs at 30MPH until I put relief valves on.

Ok, why would not having a second pump raise water pressure ?? mine work fine..with by-pass and 140 therm and a few small holes..

Hmm.. I have seen tstathousings with just 1/2" bypass. Maybe thats too small?

I think the circulating pump might help regulate in psi in the block. You can have high water psi due to the speed of the boat, hull pickup design, etc. Or you can have high water psi from a restricted exhaust water flow, etc. Some boats have relief valves in the sea strainers, to help protect from psi spikes when re-entering the water. The high psi from this can POP hoses off the strainer, pickup, etc.

Most scenarios though is too high of psi in the block, which can blow headgaskets and or intake gaskets.

My thoughts on a lot of high water psi complaints, is from exhaust that restricts water flow. The merc sea pump design puts out A LOT of water. The more you restrict the flow, the more psi is going to build up. My water psi is about 20 psi wot. My exhaust water dumps are 3/4". I bet if I reduced the dumps to say 3/8", it would drastically increase water psi in my block.