I've been trying to limit myself to basic PHB for theorycrafting lately.
There is literally no reason to do this.
There is if you're feeling around for character ideas for a campaign that's only running PHB. Like, yeah, I could be using the expansions to think something up, but I'm not in a position to use any of those character concepts. Might as well keep myself in the "zone" of PHB-compatible design for the time being.
Seems to me, if you're in sight of land it's basically coastal sailing. I'd consider extending it beyond that to be houseruling. As for whether it's OP, depends on your campaign. If you're doing a super salty sea dog pirate campaign that only comes on land to bury treasure or exchange gold for rum, yes it's OP. If most of the adventure is on land and boats are only for transport, then definitely not OP. If you're doing a Voyage of the Dawn Treader type thing, then it depends how much the importance of terrestrial content is compared to marine encounters, but it should be fine since you can still use magical phenomena if you have something cool that requires the party to be temporarily lost.
Well, favored terrain is always going to be of variable usefulness depending on the campaign, but most of the other terrain options are a lot more dynamic as far as inserting them into the setting. Forest, grassland, mountains, swamp, all of those are standalone environments that can be easily plopped into the path of the party. Desert and arctic are more climate-dependent, but both can still count for very large swaths of terrain. Underdark is a bit bizarre and is likely going to be all-or-nothing depending on whether or not you're in an Underdark campaign, but I suppose there's an argument to be made for classifying large caves as "Underdark".
Coast isn't really an environment, it's a transition between two environments. It'd be like if you had a "foothills" favored terrain and got a bonus in the region between mountains and grassland, but not in either of those places on their own... Except foothills would probably still cover a larger area than "coast". In and of itself, favoring coast terrain basically means you're really good at traversing the beach. Not the ground above the beach, and not the water beyond the beach. Which is extremely limiting as far as what you can do with it.
Would following a river count as "coast" terrain? That would help give it some use elsewhere, but that doesn't really seem to make sense with the name. Presumably you could count the coast of a large freshwater lake and not just be locked to saltwater transitions...
If we consider "coast" as also applying to journeys at sea, then it's in the same boat (ha) as the Underdark specialization: It's either massively useful because the game spends a lot of time in that terrain, or it's useless because the campaign doesn't deal with that area at all. It's not really something the DM can just "slip in" on the normal route to give the ranger a chance to be useful before moving on with the scheduled programming.
If we
don't consider coast favor as applying to the sea, then it's worse than Underdark. If your campaign featured time at sea, you'd only have a bonus when getting on or off the boat, you wouldn't be able to help out either inland or at sea. It'd be like an Underdark-heavy campaign where you're especially effective right at the transition between the overworld and the underground, but not beyond that line in either direction. Neither inside nor outside, but straddling the middle.
Basically, Favored Terrain: Cat.