A simple power jig saw will cut wood nicely though a plain old cross-cut saw works just fine - practice a little with scrap wood if you haven't much experience cutting wood, draw on your mark in one direction only until you get a solid start and only then get going with the back and forth sawing motion. If using the jig saw, rig up a "fence" to cut a straight line - but you will still have to battle the likelihood of the blade bending while cutting, bit of a PITA. Or use a manual powered (muscle) jig saw, they work just as well but slower. I use an overarm radial saw and a table saw plus my electric jig saw but a lot of people don't have the space for such tools.
There is - or was - a Stanley brand saw with a handle and pretty low teeth to length ratio (big teeth) that I use for cutting insulating foam but it is still a great mess. If you are going to buy any kind of power tool get a really good shop vacuum for cleaning up the horrendous mess that results from cutting foam. Very serious. And never - EVER - buy beaded foam!!! And don't let anybody give you any neither. An even worse mess by a factor of ten or more.
A recommendation if you decide to travel a similar path for a future project where you need a 'deep' river: buy a thin sheet of the insulating foam and use a 'cookie cutter' approach to cutting it to shape and then glue it to your thicker 'core' board. That will yield a nice flat interior to work with - much easier to seal and then paint before pouring any resin.
Hope that helps for future projects and feel free to ask questions for any clarification if needed.
I applaud your use of deep sides for this project to allow it to work double sided. A very nice touch.