Unless you have booked into a hotel on Seacoast Boulevard and plan to spend all of your time at the beach, a car is the most convenient mode of transportation. Parking can be dear, but most of the time it's easy enough to find a space for your car. The main concern is that Seacoast Boulevard is not very long and the whole length of it is well patrolled so illegal parking is noticed easily.
The colored curbs have meaning that a driver should know.
Yellow is for unloading only. Stopping for a short period is okay, but no parking.
Red is for Emergency Vehicles only, no stopping or parking at all.
Green is for timed parking. Usually 15 to 30 minutes only.
Plain concrete curbs are free and legal parking spaces with a time limit of 1 or 2 days. There is a couple of paid parking lots but no metered curb parking.
It seems that a car is the most effective way to get to Imperial Beach.
Driving I-5 or I-805 (towards Mexico) take the exit to Palm Ave or to Coronado Ave (from I-5) - drive west to the end (= to the Pacific Ocean).