Dear WW: I suspect that the seaweed could cause the greater part of the drag that slowed
the ships. In any case, removal of the seaweed would have been the first step. The barnacles
would be much harder to remove. I didn't find anything about the specific measures needed
to remove them. My guess would be that it would involve "holy stoning" - rubbing with the same
stones used to clean and smooth the decks. Then of course, the wood was painted with a mix
of tar, tallow, and sulfur.
In modern times a special tin compound, the name of which I have forgotten, was very effective
in prevention fouling of ships' bottoms. But recently it has been discovered that this tin compound
is a serious environmental pollutant. I'll try to find name of the stuff, not sure I can.

Edit: As I thought, I could not locate the name of the organotin paint that was very highly
acclaimed, until it was discovered that it was a serious environmental hazard, killing may things
other than the ship bottom fouling organisms. Not clear what has replaced it. But with very big
ships, the loss of speed costs very big bucks. So a lot of research will have been funded.