My family had a professionally built rowing shell when I was a kid.  It appeared to be approximately 3 mm ply.  It did not have fiberglass on it.

Camper We regularly use 3mm Okuome to build our shells so it should be fine,but you talk of tight curves in the bow and stern most shells are fairly gentle except for the twist of the bottom panel which is easy to acomplish What design are you building ? if your design calls for 3mm I would say OK if the designer has specified other scantlings you might want to consider following his or her spec, it seems that a lot of people are concerned with ultimate weight of there shell and to give an example of the weight savings our 22ft s&g shell built with 4mm bottom 3mm sides and deck is 37lbs without the rig,the same boat of all 4mm for durability is  39lbs this is a 22' by 21" shell very simalar to the CLC oxford

cfoxwoodboats.com

Thanks, I think I'll go with the 3mm ply.  The design is a custom design I put together I'm trying out for the team I coach.  The bottom is rounded like a racing shell and tapered in the bow and stern, also like a racing shell.

I can't get the 3mm ply to bend around the 15cm radius very well.  Are there any tips or tools for doing this?  I read somewhere that you shouldn't steam plywood for boat making.  How about planing it down to 2mm? Or kerfing, can that be done with such thin ply? 

When I was trying to make some tight bends I sprayed the plywood with hot water or put hot damp rags/towels on the wood for a little while. Then I used a lot of clamps, even making a wood form and using screws to help bend/hold the plywood for awhile. The hot water only raised the grain slightly but it was easy to sand smooth.