The check-in time says 3pm - 11pm, but when I was there, someone was at the desk to check me in at 2 a.m.! Make sure you bring your ID/driver's license for check-in.
And by the way, there can only be one car (that's registered with the hotel) per room, and only that car can be parked in SeaRock Inn's parking lot; otherwise, you risk getting towed. There's a lot of street parking nearby.
Units are secure; you can't get into the unit without a specific key card (2 key cards are given to you when you check in).
In the unit, there is a good-sized TV with cable, a work desk, 2 comfy beds, and one full bathroom.
Overall, my experience here was good! I would recommend this place.