That'll be barnacles and lack of maint. Seawater is a fluid with a mass of more than 1 Kg per 10cm^3. I have customers that worry about hull shapes etc. Increase the coeff. of friction a small amount has a massive effect. Can't remember exactly now but I think friction is proportional to the square of ... something.

The difference here is that these little beasties magically make a ship run slowly. I would bet real money that their invention was to cover up late delivery or whatever. Basically, an excuse that no land lubber of the time could possibly refute.