The del is great. But it's an old hotel. There are a fair number of very small rooms, and for one of the very few hotels on the sand in the SD area, there are a surprising number of horrible view -- some looking right into other guest's windows. It's beautiful though. I don't love the Coronado beach, though, as much as the beaches up north, like Del Mar and Carlsbad. Particularly during the summer you can get some iccky smells and stuff floating up from Tijuana.
Stay on Corondao and not rent a car? No way. How? Unless you plan never to leave the hotel, but even then cabs to and from the airport, and to and from the Amtrak station, will pratically be the same price. Plus the cost of having to eat all your meals at the del or right around it because you can't go some place off Coronado that's a little less expensive will easily cost you a mint.