I've tried googling this, and I'm maybe I'm just dumb, but I'm not sure I've found a solution.

I have a boat in a marina. With my slip come free wifi. I assume the router is in the marina office, and the signal at my boat is weak at times. It's mostly just my phone that I have connected to it, but it cuts in and out constantly.

Is there an antenna or something I can get to better grab that signal, or should I be talking to the marina owners about getting a better router or something like that?

The "best" thing to do would be for the marina owners to re-position, turn up power, and/or get a better antenna. Based on nothing, I imagine they probably won't.

Since this would be for your phone (and not a stationary device like a desktop PC), your other option is to get a wireless repeater or range extender. This option can be suboptimal because it can negatively impact everyone else that uses that wifi. If you do end up going this route, try to get an extender/repeater that receives whatever channel the marina owners' wifi is on, then repeats the signal inside your boat on a different channel. That at least used to be a harder to find device, but it may be a bit more common now. That would mitigate negative impact to others.

How would I go about finding out what channel their's is on? Definitely not looking to screw over others, just want to not be killing data while chilling at the dock.

Download an app called WifiAnalyzer, which is open source and free, or a similar one. It will show all wireless access points in range in your area and which channels they are using.

an extender/repeater is worth a try. It might not be enough though, if the signal reaching your boat isn't strong enough. Some routers can be setup to do this but not all. I wouldn't get too hung up on the channel until you know it's going to work for you.

Think you should talk with the marina owners. Depending on if is "free" wifi or just call it free but they have included the price for the wifi in the slip cost I would say it is in their interest that the wifi is good. Also before you put up extender/repeater check with the owners that it is ok.