The only reason I have ever heard for keeping the engine down was the zinc in
in the trim tab and leg but most also have a zinc on the bracket. On a pontoon
you should also have a zinc (or magnesium) on the pad in the back of the log
(usually where the speedo pitot ends up)
I vote for keeping the engine trimmed out. That is gospel in a place that grows
barnacles but you have some marine growth there too.