Wave has obvious problems with mechanical wave action and corrosion quickly destroying any machine you leave in the waves, access and maintenance. It's unsurprising this is very difficult to scale and it does seem a dead end.

Tidal energy does seem like in principle it should work without many moving parts at all save the turbines, which don't have to be metal. Is it the turbines themselves which are very difficult to maintain in salt water?

This one is a floating design, which isn't ideal for maintenance, but I imagine at a large enough scale a dam between two bits of land with turbines in it would work - something we already do with hydro power and fresh water, and a concrete dam with access tunnels would solve a lot of problems with access and maintenance.