Won't hurt gelcoat, but can get stinky
Water ballast tanks, like bilges and sumps, are dark stagnant "swamps," full of dead and decaying sea life, fermenting in the summer heat...and like bilges and sumps, water ballast tanks need to be cleaned occasionally. Not just by throwing in some bleach from time to time (After all...you don't clean the bathtub just by throwing some bleach in the dirty bathwater and leaving it there, do you?), but actually cleaned with detergent and bleach--scrubbed if possible, but at least agitated enough to loosen all the slime from the walls--and then thoroughly rinsed out with fresh water to remove all the dirty water. Once or twice a year should do it.Once or twice a year should be enough to