I would guess a 7 amp-hour sealed lead acid battery:
Amazon.com: Sealed Lead Acid Sealed Lead Acid Battery For 12 Volts / 7 Ah: Electronics
would be able to pump about 1,500 gallons of water on a charge with a pump like this:
Amazon.com: Rule 25D Marine Rule 500 Marine Bilge Pump (500-GPH, 12-Volt): Sports & Outdoors
Put the battery in a dry box. For twice the price, you can get an automatic bilge pump and you won't need to mount a switch somewhere. If you go automatic, get the kind that turns on every few minutes and senses the motor resistance rather than the kind that works on a float switch. A float switch would bounce around too much on the river. The whole system weighs about 6 pounds.
A 5 watt solar panel will produce about 4 amp-hours of charging in full sun over a 10 hour day. Last time I tinkered around with this idea, the amount of solar panels needed were way to big to be practical, but the technology may have improved since then.