This is a pontoon with a frame. That opens up more options for what kinds of rod tubes you install and where you mount them. The big consideration is placing them where they will not interfere with oars and rowing.
In looking at the overall design of your craft, it looks like you might want to mount a few tubes along the bar behind the seat. I am attaching some pics of how others have done that.
I am also attaching a writeup on basic PVC. I use schedule 40 PVC for construction pieces that need strength. Use type 200 1" tube for rod handles...for most rods. If your rods have bigger handles you may have to go up to 1 1/4".
Also, if you look in the plumbing section you can often find clamps for 1" frame pipe with a 1" fitting for connecting pipe. Better than drilling, but you can drill and screw...without weakening the frame...if you use some silicone seal around the screw holes to keep out water and prevent rust.
HERE IS A LINK to a post I put up a while back on completely pimpin out a pontoon...by drilling and screwing the connections to the frame. Some folks take exception to it but I have never known a frame to fail or rust because of it.
The key thing on using the frame is to have a good drill bit for the pilot holes...and use good machine screws. I like 3/4"...8 or 10 by 32.
Have fun and don't hesitate to post any questions or send me a PM if you prefer.