MC4 is great. Been in place since 2016 and never any issues. Cheap too. I put a splashproof junction box for gardening electricity connections in the stern lazarette to join the cables entering from the solar arch with the ones going further into the boat (to solar charger, antennae, etc.). That way I can undo the joints in that box if I ever need to remove the cables (never needed to so far - knock on wood). Would avoid plugs, as good ones are expensive and still prone to break (accidental knocks/kicks or UV damage). Only do that for removable installations. Deck glands, swan necks or in case of many cables, a piece of flexi conduit rigged like a swan neck all work well for routing cables inside.
As usual, coat any connectors in a bit of silicone grease/spray before assembly to avoid moisture ingress and contact corrosion. Works a treat.