Capecodder, out of the three you listed, I'd say NRM. I had my mooring and slip there for years. The Zarella's are decent folks too. Kings landing is a bit too far upriver for my liking and James Landing is way too much $$$. The other alternative is Mary's Boat Livery. No slips there. Just moorings and floating docks. They'll ferry you out to your boat. I'd still go with NRM though. Just prepare yourself for the No-See-Ums and green head flies
The mouth of the river can definately get a bit treacherous at times like an east wind with the outgoing tide will stand those waves right up. Just got to learn to work around tide and wind conditions like that. Beamie keeps his boat over at Mary's so if he weighs in on this one, he'd be able to shed some light on their operation. I like the area and even though I trailer my boats now, I still frequent there a lot.