When launching rockets on land you need flame diverter trenches to deal with exhaust and water deluge systems to deal with the sound (the water droplets absorb sound). When launching on water, dealing with the exhaust is more straightforward, but sound travels much better in water than in air, so it's a much bigger problem (mostly for marine life). One idea is to create a curtain made out of air bubbles that effectively blocks sound. The bubbles force sound waves to transition from water to air and back into water, and as you know from putting your head underwater in a pool, sound waves don't like doing that in either direction.