Why hwacha were risky aboard ships

How extensively were hwacha used in naval combat?

They may have been used, but probably not often because their rockets could set the launching ship on fire.

A shipboard rocket array could provide significant firepower, yet any malfunction threatened the wooden vessel carrying it. Safe employment required space and careful handling, which reduced its practicality in a crowded battle fleet. Naval use remains possible but appears uncommon. Specialized rocket vessels in other navies often stayed outside the main battle line or supported sieges.