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Mountain Tops manages the concession down at Long Dock. Weekdays, you can call or walk into their shop to arrange a rental, a tour, and/or a lesson. On weekends during the "season", Mountain Tops staffs the facility and handle rentals and everything else at the pavilion.
Mountain Tops has a range of boats from recreational 12' to 16' touring and sea kayaks. The Deltas and Walrus kayaks they sell are particularly nice relatively low-cost independently manufactured boats.
Full disclosure: I give kayak tours for Mountain Tops.