Get a car and then park at Silver Strand
Having not being able to afford the Loews back when I was a grad student also for a conference, I'd find a cheaper place to stay on the San Diego side with free parking and then drive over the bridge and park across the street at the beach. A bit more walking but a lot more flexibility.
I've stayed at the Marriott Coronado more recently (about a year ago) and, even as a Platinum, really don't recommend it. Overpriced rooms, limited (also not free) parking options, no real benefits as a Platinum, and really, really old furnishings made this a miserable stay.
Any place in downtown/gaslamp, hotel parking is going to be starting at $30 but you'll be in the heart of the town. The Loews is literally nowhere and, without a car, you're stuck there.
David