Claire, the closest beach can walk to is the Harbour beach (Liminaki beach?). Out of the hotel turn right and follow the road straight for ten - fifteen minutes walk and your at the harbour. The beach is very large and beautiful spreading out along to the left. You are very centrally located so Napa's other beaches are all further away. Sandy Bay would be a good 25-35 minutes walk down Nissi Avenue (turn right at the traffic lights/Tommys Bar - or there is a path along the coast from the harbour). This is a smaller, quieter beach. Nissi Beach is another 10-15 minutes further along (Lively, one big party!). Landa and Macronissi beach are even further towards the waterpark and would entail a bus trip or taxi ride (Landa's worth it though, beautiful, quiet beach but with watersports and a couple of bars - just seems so much more tranquil).
Never stayed at the hotel but it is ideally situated and from what I have read about the place because it is set back from the street it is a lot quieter than first impressions would suggest. We have considered staying there but always end up staying at or near the Thalassaki Village.
There's no shortage of boat cruises, you'll be given leaflets every time you go within 100 yards of the harbour (no hassle, just leaflets) so shop around till you find what you want (there's the party cruise type with games etc or ordinary cruise/swim/eat).
The square is just five minutes away from you but within steps of where you are there are friendly 'family' bars - -The Golden Arrow, Napa Star, Tommy's, More 'sophisticated' cocktail bars - - Pepper, The Lobby Bar and all kinds of decent restaurants.
Have fun!