I will second the 'straps, lots of them' message. Anything that is not well tied in will some day be an offering to the river gods. No one has too many straps.
You might consider one of the 'everything bags' for the cargo bay. These are big mesh bags that are tied into the boat and have a drawstring closure at the top. Instead of tying in lots of small stuff individually, you just dump them in the bag and make sure it's well tied. Great for odd shaped things that don't come with tie in points, like a propane canister.
The Revolutionary Everything Bag
If you do a lot of fishing I would consider some rod holders. Rivers are really hard on gear and rods stored loose on the boat will soon be broken or go for a swim. I use some 'scotty' brand holders bolted to the frame for a place to park them while actively fishing.
Scotty 265 Fly Rod Holder with 241 Side / Deck Mount
For longer term storage (like over night) I have a couple of 10' sections of PVC pipe strapped to the side rails. I cut a 1/2" slot the full length of the pipe so the rod can be slipped in or out one handed. Rods left loose in the boat after dark will be located by the snapping sound when you go back to the cooler for that late night beer.
I use a dedicated ammo box, strapped to the frame, to carry my fly boxes. I can flip open the top and quickly grab a box, then dutifully close and lock the lid. I once saw a poor fellow lean over too far and drop three fully loaded streamer boxes out of his shirt pocket and into the drink! He lost about 200 cone head streamers each worth $2.95 in that 10 second mistake.
A good quality boat net with a handle long enough to reach out over the tubes.
Fishermen need their hands free for knot tying etc. A set of oar leashes have saved me several times when the oar blades hit something while my hands were busy.
I bought a pressure gauge with my first boat, I've maybe used it twice in 10 years. So you probaly don't need one once you get a feel for proper inflation. A cheap air blower for quick inflation at the put-in will be used often. There are some bomber ones for around $150, but mine is just an old 110 vlt electric leaf blower with a 12v inverter I bought for under $30.
Put together a good 'spare and repair' kit. I carry a spare valve, a spare oarlock, a patch kit, spare valve adaptor for the pump, a small collection of tools for repairs. You might want a 'z-drag' kit for getting off rocks, esp. if you are floating rivers with light traffic.
Consider a trailer. It's awsome to be able to do multiple single day fishing floats and leave the boat fully rigged. You can be on or off the ramp in just a minute or two while the others are inflating or deflating their rubber.
Make sure what ever PFD you invest in meets the river requirements. Some rivers regulations and rangers are very picky about what the labels say.