They will probably use Marine frequencies, but if they work with lowland rescue, or do foot searches of the river bank will probably use those too if they use radios. The fact is, if it's in a good & reliable mobile phone reception area, they could use phones, as a cheap & reliable form of contact.
Also try PMR General/light as again,it is a cheap solution & people can buy their own radios & everyone can use under a cheap licence.
The only real way to know is to band scan when you know they are active.
If others have not done the fun part, then you have the opportunity to find this one & add it yourself.