My boat has four batteries, two with a switch for starting and running the electronics, and two more completely isolated from the boat's electrical system that are for the trolling motor.
Most saltwater boats will be set up with two starting batteries and use a switch like this one that allows you to choose battery 1 or battery 2 with a third position combining them in case you're having starting troubles. I use battery 1 for one day then battery 2 the next day, you don't use the combined position unless you're having trouble.
https://www.amazon.com/Blue-Sea-900...mp;psc=1&refRID=QTPFRYAVFMET0WPE39FETwo batteries are better than one for redundancy, I wouldn't want to take a boat with one starting battery very far away from land.
There are three general types of lead acid marine batteries, one meant only for starting, deep cycle batteries means for high draw things like trolling motors, and dual purpose batteries that are about halfway between the two. Mine are group 24 dual purpose types (group 24DC), they're for starting the motor and also run the electronics like the GPS/fishfinder, stereo, pumps, etc. The two trolling motor batteries are type 27 deep cycles (group 27DCX) and only run the trolling motor. My batteries, both the starting batteries and deep cycle batteries, are duracells from Sam's Club and I've been happy with them. They're about $80-90 each. The expensive batteries you mentioned are probably AGM or lithiums. Only you can decide if they're worth 4X the price, they're not to me.
Opinions vary but I'm of the opinion that the full time trickle chargers do more damage than anything. People complaining that their batteries don't last usually start their post with "the batteries have always been kept on a charger". I never charge my starting batteries unless it's the beginning of the year and the boat has been stored for the winter, the boat's charging system keeps them charged. My trolling motor batteries I charge fully when I get back from using them then again before taking it out if the boat has been sitting for more than a couple of weeks. I don't keep them on a constant charger.
What would I recommend for the OP's purposes? I'd recommend two dual purpose batteries 24DC batteries wired with one of the aforementioned switches, that'll keep you out of trouble and won't break the bank. Many of the high dollar designer batteries are overpriced junk.
