Why a modern yacht is a fragile warship conversion

Could a modern yacht be converted into a warship by replacing leisure spaces with modular weapons?

Only in a limited way: the hull is too fragile for major modules without heavy reinforcement, although some light non-penetrating systems could be added.

A modern yacht's light structure is the first obstacle. Removing leisure spaces does not give the hull the strength or weight margin of a warship. A StanFlex-like module would require significant metal reinforcement in the hull. Limited non-deck-penetrating systems might still be fitted, but the result would remain an auxiliary conversion rather than a robust combatant.