I guess that the anti-Sea Lion action would have been a one-off sacrificial op by the RN, carried out without regard to losses. On a scale of national importance, it would have been up there with the Armada and Trafalgar. =
No, RN losses could have be relatively minimal, it being perfectly possible to leave Pompey at teatime, nip along to Eastborne, brass up ze Geermans, and be back home for a fry up for breakfast with nothing much the Germans could have done about you. Lest we forget, the RN ships was regularly shooting up French ports at night in September 1940.
However! Without RAF air superiority, the task of the defenders on land would have been much harder if the Germans had gotten ashore.
The RAF were just as necessary as the Navy. You can't fight a modern war without air superiority.