Oil Recommendation & Old fuel Issues

[SAMBA]

I have a 1953 33' Sedan Cabin Cruiser, twin KL flat head sixes. I'm doing more of the maintenance myself for cost reasons. First question is easy. Last year my mechanic used synthetic oil at the oil change, my assumption is that's expensive and unnecessary and possibly harmful. Just confirming, straight 30W non-detergent oil is best for those engines?

Next, my boat had partially sunk in 2019 and my recollection at that point was that the marina had suctioned out the gas tanks and replaced with fresh fuel. Present mechanic thinks that may not have been the case, getting lots of water in the fuel with lots of debris. Recent change of fuel/water separator revealed lots of yucky stuff as well as significant amount of water. Recently the starboard carb had to be rebuilt because of rust and debris. Definitely need to fix it before next summer, boat was not running well this season. Boat will be hauled out of the water in two weeks, mechanic is recommending cleaning the gas tanks and flushing out the lines. I've checked YouTube, lots of advice for how to set up an actual filter system to clean the fuel in the tank but my situation is different. We need to get all the old gross fuel/water removed and then would be nice to clean the tanks as much as possible. I've seen a recommendation to drain the gas first and then to use straight alcohol soak for a day or so and then suction that out. Another recommendation was to suction out the tanks and then use a pressure washer to spray water into the tank and suction that out and dry it. That seems like it could be a lot of work and maybe not a great idea? This will probably need to be done in place, the tanks are pretty solidly built in, I can't imagine trying to remove those big 60 gallon tanks from below the back cockpit floor. Wondering the best way (the most economical) to clean out the tanks once we have sucked out the old gas. By the way, I do only use ethanol free premium, so no ethanol in the fuel supply. I have not looked closely, but wondering if there is typically a drain at the bottom of the gas tank?

Thanks so much, Dan (Samba)