Complex sentence: Bottom paint improves ship performance and durability as it reduces the rate of biofouling (the growth of organisms on the ship's hull), but the TBT slowly leaches out into the marine environment where it is highly toxic toward nontarget organisms.
Split into simpler sentences: Bottom paint improves ship performance and durability as it reduces the rate of biofouling (the growth of organisms on the ship's hull). Unfortunately, the TBT slowly leaches out into the marine environment where it is highly toxic toward nontarget organisms.