There is actually quite a bit in Guatemala and neighboring countries.Boats or guide books are hard to come by though.Lanquin is the boating hub and a cool little town with a couple natural wonders nearby.It' s very inexpensive and there are several rivers in the area ;Cahabon , Lanquin,Chixoy,Polochic,Sauce,Matanzas/Panima,Chiyu[Las Conchas],La Pasion,and others that have been run.The Cahabon and Lanquin are the most commonly run.
Maya Expeditions [ google] has some beta on a few other rivers on thier site; Motagua,Coyalate , Nahualate,Naranjo, and Los Esclavos,mostly on the west coast.The Pacific stuff runs late summer early fall.Google Huehuetenango rafting, there is an outfitter on the Cuilco and other rivers in that part of the country that have been run;Cabuz, Suchiate,Ocosito,and to the east the Copon in badass wilderness 4+ with creeks to explore in the headwaters.The Rio Paz,the border with El Salvador has 2/3 and class V sections on the way to surfing.The Usumascinta, on the border with Chiapas, is a big water 2/3 multiday with multiple archeological sites.
Most visitors to Guatemala want to go to Tikal ruins ,the most impressive Mayan city and a nature park,in the Peten.Peten is flat so not many ww rivers,but near Belize the Chiquibul and Mopan are 2/3 rivers.Not far inside Belize are the Macal and a couple tributaries with good whitewater in season.If you splurge and stay at Black Rock, Blancaneaux,or Five Sisters Lodge[s] you will have good rapids/drops at your doorstep to lap,huck ,or explore above/below.There are other rivers and sea kayaking on the way back to Guat. if you go the Caribbean route.There are rivers near and in Honduras[has a guide book].
Guatemala is centrally located beween Hond., BZ.,ES., and Chiapas,all a cheap 1/2 day bus ride away.All of them together are smaller than California and good sized chunks without boating could be lopped off.Tons of boating but few resources for ww boaters with the exception of a few outfitters.La Ceiba and Lanquin are the best bet for boaters without equiptment.
There is a guy in Antigua who really knows the country well ,numerous 1st d's,if you are serious PM me I'll get you his contact info.