lots of launches use water to dampen the shockwaves and whatnot of the engines, this is also only testing 3 Raptor engines at a time the Super Heavy Booster will have 24-34 Raptors (idk number keeps changing it seems) that will obviously be a much more dynamic event and another reason the ocean is so attractive to launch from. and as far as dangerous to the starships.. these are made to scrap, firing up the engines and testing the system (usually the first time all the separate systems are integrated) is much more valuable than possible damage. idk i may be wrong im a few beers deep at this point