How much is too much and how much is not enough? I actually think the ride is sometimes wilder when the water is LOWER. It really all depends on the particular stretch of the river you're on at any given place and moment. One thing that is for sure is that the rainy season brings much greater flow and that means faster water current overall. However, it also means higher water levels and that can mean at some points most of the water flowing OVER the rocks rather than AROUND them. In a large inflatable raft that means that you more often bounce harmlessly over them if you hit them at all then you would if the water were a little lower. Sure, you travel faster but is it as exciting as when you have to respond quickly to steer around the rocks (or bounce back and forth between them like a ball in a pinball machine)?
There ARE some rivers where they become unrunnable at certain times of year. For example, the Naranjo River near Quepos is only run from May to December. I also believe the Class V San Miguel section of the Sarapiqui is also only runnable (by rafts) during the rainy season (though I might be wrong about that). Also, you should be aware that even rivers that ARE runnable year-round may have different difficulty ratings at different times of the year (e.g. class IV for a particular stretch one month and class III during another).
This was all probably more than you wanted to know about rafting in CR, but I'll throw out just a little more advice to address your particular interests. You indicated you're planning on going on this tour largely as a "photo op" I assume so that you can show pics of you doing other things to folks at home other than consorting with prostitutes. Rafting is definitely a fantastic option for this. HOWEVER, you should be aware that "self-photos" while you're in the heat of the action (the ones you'd be most interested in) are next to impossible. The rafting companies know this and that most of their tour participants would like to have photos of themselves running the rapids (not just the ones looking for an alibi), so they have a professional photographer who positions himself at key points in advance of the rafts and takes the photos of everyone. The catch is that those photos always cost EXTRA, nearly always quite a bit extra. Some outfits will offer to sell you a nice 13X9 of just yourself (for $10 or more), but what most of them have taken to in the last few years is trying to sell you a photo (or even video?) CD of your entire tour including all the OTHER participants (ie 95% of the photos you're paying for you have little or no interest in). There will definitely be a number of decent pics of just you or just your raft group on the CD, but you'll have to purchase the WHOLE CD and they'll probably charge at least another $30-40 for that. I suppose if its your first raft trip and you don't have any other photos that could still be well worth the added expense for you. If you're a cheapskate or a guy who has been on many raft trips and already has plenty of pictures like me, then that CD may seem a bit too pricey. You COULD see if one of your fellow tour participants is interested in splitting on the CD (with one of you promising to send a copy to the other). If they're purchasing the CD anyway they may even be happy to e-mail you the pics of just you or your raft gratis (although if I were depending on these for my "alibi" I probably wouldn't want to rely on them to remember).
Another thing that you might want to do even if you plan on buying the CD at the end is to bring your own camera, which you can pull out to use during the more placid stretches of river as well as during your lunch stop on the riverbank as well as before and after the rafting portion of your trip. I often do this. If you decide to go this route, I recommend you DON'T bring your expensive camera. I always use a cheap disposable WATERPROOF camera for this sort of thing and it usually takes pretty decent shots. Think very carefully where you try to stow your camera. You'll want it to be readily accessible (if you plan to take any pics while ON the river), but you also don't want it to get in your way (or worse come loose and get lost) while you're paddling through the rapids. AND don't count on taking ANY photos during those hectic moments.
All told, some other type tour might be easier and more economic for purely photo op purposes. For example, you don't have to worry about your camera being waterproof when on a canopy tour. Stowing the camera so that it is secure and out of the way but still readily accessible (while wearing thick leather gloves

) is still a concern. Also, as with the rapids, taking pictures WHILE you're busy zipping down the cables will probably be completely impossible. But it is much easier to take your own action shots or at least hand the camera off to one of the guides or other tour participants to take some of you while you're flying through the air will be much more practical than the similar situation while rafting. The pricey marketing (upselling) that is standard with the rafting companies, hasn't seemed to have taken over so much with the canopy outfits and it is much more common to "do-it-yourself".