Haha, concrete. I have a friend who built sailboats here and swore up and down that concrete makes the best sailboats. Hard to believe, but he showed me photos.
I just recored Some decks and made some hatches for mine. I used 1/2” Finnish birch plywood. You can get it from a wood store not Home Depot or Lowe’s. The Finnish just means (more or less) that you have a substantially higher number of plys vs traditional plywood. I’m very pleased with the results of it, so far.
11 ply’s I believe on this one For around $50-60 for a 1/2” 4’x8’ sheet
The teak 3/4” 4’x8’ at $280 was only 7 plys. The maple 3/4” was 7 plys as well at $110/ sheet. That’s prefinished cabinet grade plywood. You’ll find the big box stores are even lower counts.
Thermolite (coosa) would work well too. It’s lighter, water resistant, and bugs won’t eat it. End grain balsa may be a good choice for lightness (Over plywood) but doubt you will notice a difference In sturdiness.
Good luck!