Are you on Facebook? There's a UCSD housing group on Facebook where you can ask about summer housing. A lot of students are in year long leases or go home during summer and start their lease early summer. They rent it out to people usually up front or so.
I've seen two bedroom Costa Verde apartments close to UCSD go for around $1500. And that's a VERY good neighborhood. Mira Mesa is cheaper but it's also a LITTLE sketchy. Not in ways like, armed robbery or drug dealings. More like, people will vandalize your car or something. You'll see like, late teen "Asian Gangsters".
Don't do vacation housing. The prices are gouged. And don't go through an international renting service. At the end of the day, even a cheap hotel on Hotel Circle will be a 10 minute drive from the beach and those places are like $40 a day. So set the bar around that (even in summer).
Being close to the beach? You could get something in Pacific Beach. You'll be about a 10 minute walk from the beach and about a 7 minute drive (massive amounts of traffic lights). Forget about getting single rooms for each of you. Not gonna happen.
San Diego is the city of two bedroom apartments. That's literally like 90% of the rentals here. Also you won't find a house/condo for rent anywhere inside the major city areas. Unless it's expensive.
But if you don't mind it being an apartment with WiFi, probably A/C and maybe a washer/dryer in the unit? You can have it for $1400 in Mira Mesa, Pacific Beach, some parts of La Jolla, La Mesa (Not close to the beach). If you guys want college tail, go to La Mesa because SDSU girls are easy UCSD girls are NOT easy. They want boyfriends, not one night stands and they know international blokes aren't good for that. Beware though. They call it STDSU for a reason.