I would honestly consider talking to your friends and maybe sit this out until you're back up to a better strength level. The last thing anyone wants is for you to exhaust yourself on the water. I'm not sure what kind of boating you're doing, but it's putting yourself at risk and putting your friends at risk too if something were to go wrong in the middle of a body of water, especially anything with a current.
I'd recommend you dedicate time to physical therapy followed up by a regimen directed at the issue. Gyms have personal trainers that will focus on what steps you need to take and make sure the exercises you're doing are the right ones. Losing muscle happens fast, as you've probably noticed. Building muscle is not nearly as easy and will require dedicated routines and time, more so the older you are. From what you're saying you seem to just lose strength and your hand strength goes away, which is pretty normal when you're rebuilding muscles or just getting started building new ones that haven't been used in a while. After surgery it's pretty common to lose muscle mass and strength and it's never an easy thing to regain. Usually when a real strong person goes through that it's much harder to get back to their previous levels. The worst thing you can do is stop trying though, it just takes time.
Grip training is fine and is easy to do, but only applies to a certain group of muscles in your hands in one action and when paddling grip isn't the most important thing. All grip will do is prevent you losing the paddle. It won't really fully prepare you for paddling as you'll be doing constant push-pull motions that put strain on many different muscles in your hands in different ways. You'll be twisting your wrists to steer and turn the paddle, but the majority of the strain will be in your shoulders, upper back, torso and your legs which you'll be using to brace yourself constantly. I would recommend taking a small weight (like 3-5lbs), and try to hold it upside down with just your fingers and no support from your thumb and try to keep it up as long as possible multiple times/day. You'll lose strength and eventually won't be able to do it which is normal when working new muscles, take a break and try again later. Over time it will become easier, it just takes time. You'll feel it in your hands and extensors in your arms which is exactly where you'll feel the burn when paddling. Doing this while holding your arm out to your side will also incorporate your deltoids and triceps which are used when paddling.
If cardio/endurance isn't an issue, maybe try a cardio exercise that includes your hands.
Off-road cycling actually does a lot for the upper body and your hands as you try to keep yourself steady over undulating and unpredictable terrain. It also works the hands in both directions targeting many of the same muscles as paddling. I have pretty big gorilla hands and a ton of grip strength and am overall just really strong because of what I do for a living but after a long time on my bike my hands are sore because I'm using them in a much different way than I'm used to. It's a good starter for building hand strength too because if you start getting too tired you can just back off and ride normally.
The other really good exercise for hand strength which directly pertains to your activity is a rowing machine at a gym. This will give you the push/pull you need, work your shoulders and arms as well as your abdomen and legs. It's a really great exercise all around.
Rock climbing offers another great overall workout as it involves all the muscles you'd use on the water as well. Handholds of different sizes require different hold techniques, you use your upper body and legs to stabilize as you climb.
Pull-ups are an easy (and free) way to build upper-body and arm strength and switching up hand positions will do a lot for all the muscles in your hands and forearms, even more so if you don't use your thumbs to aid grip (hang from your fingers)
Two weeks may not be enough time to really make any significant changes before your trip. It all depends on how quickly you heal each day and to an extent how far you need to go. I just recently started cycling again after almost 30 years and on day one despite being really strong, I almost couldn't make it up a 1 KM long hill because I never use those muscles outside cycling. It was exactly as you describe, the muscles just stop working and I have to stop. I heal up fast but it'll be months before I can ride my bike half as easily as I could when I was a kid, it all comes down to practice and patience.
Whatever you decide to to, just remember to keep at it and make it routine.
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