If you are within sight of each other and the river/lake is void of constant boat traffic (noise), a whistle works great. So does a small air horn.

You can purchase FRS/GMRS radios that are waterproof just about everywhere now, including Walmart. That will get you about 2 miles of range.

A marine hand-held radio would be the next step...I carry them in my sailboats...because that would be where rescue authorities will be listening, assuming your lake/river has
them.

My prefered option would be for you and your wife to get your amateur radio licenses (35 question test)and learn to use the huge array of tiny/powerful/waterproof hand-held radios available. With those, you can work through local repeaters and extend your range by many miles. I carry mine backpacking where cell coverage is nil.

All that said, I ALWAYS carry my cell phone if I know there is coverage. It stays in a nice waterproof cover (about $15) and works great.