Been running boat for a while. Had two episodes where it would not start after being run and shutting off. Got towed twice and now feeling like I have water getting into cylinders. Starter motor spins but nothing happens, I can manually move the motor using breaker bar without much effort. Pulled all the plugs and it turns over fine. Plugs 2 days later were dry. Motor seemed to idle and run fine except for top end where it was around 4600rpm. but can have up to 5100 with 24P props. Speedo shows over 50mph. Pops up on plane no problem with 3/4 tank fuel and 5 people. I actually have a set of 26P props for sea level so I can get a bit better cruise speed and cruise economy (3000rpm is around 35-40mph depending on conditions).

I noticed a couple things. Motor was loosing power on top end (4200 plus) while driving. Oil level is high. Going to do oil change. RPMs fluctuated if that helps. Also motor sat for a long time and use is in Salt Water. When I removed risers inside of manifolds and risers looks very good but had to clean up rust on risers where they go to Y-Pipe to exhaust. Bottom of manifolds bad where I can't see?

Flappers? Manifolds? Something else?

Mechanic seems to think its likely manifolds leaking water in.

Motor sticker says NGK BR6FVX plugs which have been replaced with NGK BR6FS on one site and another says BR6VIX? Any insight. Motor is 454 MAG EFI GEN VI with Motor Code M00C107 (possibly M000107).
https://www.vansoutboardparts.com/me...=17,4,655,3394

Anyone use the Magnum Tuning Plugshttps://www.magnumtuning.com/en/deta...ces-ngk/br6fvx

the salt water may have rotted the manifolds&risers causing reversion,or the heads may be rotted where the valve guides press into the heads.i would not run it again till the problem is found because if you hydraulic the engine it may be rendered junk.

risers are junk, probably

I’ve removed Brisk plugs from a handful non running (for no other reason )engines. In fact, last week-end a snowmobile with a new set in it. Lol

Oh, for plugs you can tun standard acdelco mr-43t ‘s or ngk normal or platinum see here : Which Spark Plugs work better?

Replaced the plugs and oiled cylinders. Ran it briefly (No Hose). Put the the NGK BR6FS in. Ran smoothly for the brief time I had it on. Looked closely at all the joints. Saw a small area with rust at riser to manifold mating area. Might be leaking there are manifolds at the base. When I pulled everything early 2020 it looks good. Ran boat for about 30 hours since. Recently started getting issues. Guess I will be hunting for either performance manifolds or 496 manifolds to replace my setup.

If I am pulling the top end should I do any modifications. I have a bit of soot in the engine building up. Might be running rich or associated with some water getting into cylinders and causing mix to be off.

Guess next step is pulling manifolds off and flappers.

Sorry for the basic question but never done head type work. What do I need to take apart to examine heads?

Getting 496 Manifolds or Stainless Marine and will see how things go.

Sounds to me like you are doing a lot of guessing as to what the real problem is.

You may have ruined your sea pump impeller by running it "briefly" without being on a hose.