Quick video I took of both motors idling with the silent choice "on" to keep things quiet. Note the water churning behind the port motor.

That's what I was looking for, you did your due diligence.

Head gasket

or burned a valve

I try and do as much research as possible before making changes. In a previous thread, comments were made that the motors were likely extremely rich with my previous jetting. Constantly checking the plugs last season, it confirmed most peoples thoughts. Having not wanting to change anything last season in regards to jetting, I waited until winter "break" to look into it further.

Based on the Holley factory jetting, HP500 jetting, and a lot of other peoples' builds (502s, HP500s, 540s, etc) lead me to jet down to the 79 primary and 89 secondary. Again, today was more of a test and tune anyway, and was just a first season check and I was not hard on the boat.

I monitored all gauges throughout running the boat and nothing was out of whack.

All plugs are the same and what come recommended for the DART heads. Plugs were gapped 0.040, which was the same as the plugs that were in it before. Same model and everything, just new.

Port eng plugs appear to be steam cleaned. Looks like water is getting in from somewhere. Have you cranked it over with plugs out to see if water shoots out any of the holes? Need compression test results.

I believe you have a bad head gasket or leaking header/exhaust manifold.