I don't know whether it's Guernsey in particular but when I cleaned mine the other week I noticed a large amount of jellyfish remains stuck to the hull!
This and the obvious algae bloom season might have a large impact on performance. For some reason in Guernsey the hulls seem to get a thin layer of algae slime that can cut the performance down dramatically.
Take it out of the water and spray with industrial bleach, leave in the sun for a few hours and then wipe over and it should all just fall off without too much effort.
Quite a few people over here tend to just find a deserted beach and beach it at low water and give it a quick spray with bleach. Not exactly environmentally friendly and I'm not suggesting that you should do the same.
However the harbour authorities use parazone when they clean the slipways, pontoons etc. So make your own choice on that one!