> Sea-ready boats were always scarcer and more expensive than weapons in the first place

Coast-hugging boats can be made from wood with some skill. Weapons in that era requires metal in some part. That required a lot of skill.

I think you slightly underestimate how much work and expertise it is to build ships that can transport an army over a long distance on sea.

> ships that can transport an army over a long distance on sea

This isn’t what the Sea Peoples built. It isn’t even what the Romans built. Most Mediterranean cultures lacked the navigational and ship-building expertise to let the shore out of sight. That doesn’t mean they didn’t need skill. But it was less skill than needed for metallurgy, in part because the latter’s inputs are dearer.

I'm honestly starting to doubt you've ever been on a ship at sea. Being in sight of the shore is going to be of little comfort if your makeshift raft capsizes because of the waves.

Making a boat that can carry multiple people on sea without sinking when the weather goes a little sour (which can happen in less than an hour in the Mediterranean) is a very hard thing to do. And if you know how to do that you'll never have any shortage of metal or other goods in the first place because you end up being the one who trade them for profit.