I think the solar aspect, combined with the canoe aspect makes this use case a bit unique - I don't think you'd want a regular solar panel either, you want a marine one built to deal with potentially being dunked, and probably a powersports battery or larger to charge from the panel, running a car charger or inverter off of that. You should be able to get 5 or 6 charges from a bigger battery if it's full, which will give you some buffer between sunny and cloudy days. You'll want to keep an eye on the charge level, so get a cheap led digital voltmeter that just shows the battery voltage, that way you can keep an eye on the battery level and know if you're killing it - completely discharging it will reduce the battery's capacity - you'll need to know what voltages it should be, and when to stop using it before charging again.
You'll probably also want a way to orient the panel, ideally a way to slew it around to face the sun whichever way you're headed in the boat. On top of that, you won't want to get up to the bow every time you change direction to orient it, so tye some string or get a remote throttle lever like for an outboard and attach that to the pivot mechanisim so you can turn it from your seat while paddling.
This setup will be in the way a little for seeing in front of the boat, so you'll probably want a way to remove it quickly for dangerous sections of river, lest you miss the cues for rocks, and things in the water. If you put it at the back of the boat you'll just forget it's there and not orient it. Maybe you're travelling north sout mostly and can just have the panel pivot left to right. Maybe you get a big panel and just lay it flat, but you won't get much charge at any time other when the sun is overhead - pointing a smaller panel at the sun will certainly get you more charge than a panel twice as big that doesn't face the right way.
This presupposes that you're in the boat most of the time and want to charge from the boat.
Are you always in the boat, or are you boating somewhere to a destination in the backwoods, staying a week or two then shipping out? I assume you boat all day most days and camp in the evening for your trips, but if you're staying at camp for multiple days at a time and just using the boat to get in and out, get a big folding panel, put it in your drybag or barrel or wherever, and use your battery to charge along the way.
Finally, the Anafi has very similar requirements for charging as most phones, the uniqe aspect here isn't the drone itself, it's the combination of trip length and mode of transportation - the solar part, and to a lesser extent the battery bank that's suitable to be charged by solar from the boat is more of a canoe/kayak trip forum question I think - there are going to be lots of people on a dedicated small human powered watercraft camping forum that charge their phone somehow for long trips.
