To answer your question directly, you would want to seal the engine so that you can pull a vacuum in the crankcase.  This would include turning your main seals around backwards because they will now function in reverse.  Depending on how much vacuum you are able to pull, you may need a regulator so you don't pull too much and suck in your gaskets.  Your pump may have an integrated regulator.

Now, to address some real issues.  First thing I would suggest is doing a leakdown test.  If you are leaking more than 15%, you are blowing a lot of horsepower right out of that dipstick.  Ring seal should be a top priority.

If your leakdown is within acceptable parameters, I would then evaluate if you have sufficient crankase ventilation.  You really need more than just a PCV setup on a boat.  Do you have open breathers?  Are they of sufficient size?  If you have an option, I would make every effort to mount them towards the front of the valve covers, with proper baffles.

I really wish people would stop recommending vacuum pumps for blow-by. 

Thank you for the info. I will do a leakdown test. But I will have to google how to do it first. . I have two breathers. One on each valve cover. Both are towards the front of the motor. I was thinking of adding two more breathers but was told that wouldn’t work. And I don’t have a PCV.


Sent from my iPhone using SoCal Jet Boats

Typical push-in breathers?  Look inside the breather boss that pushes into the valve cover grommet.  You might see a crescent shaped cut with a slight bend in the sheet metal, acting as a baffle.  You might need to give this a little more of a bend, to open up the flow.

Are you getting a lot of blow-by out of the breathers?

https://www.harborfreight.com/cylinder-leak-down-tester-62595.html

As far as blow by out the breathers. I get a little bit of oil on my valve covers.


Sent from my iPhone using SoCal Jet Boats