If you're taking the Catalina Express over to the island, check the company's website for baggage restrictions.
There's a spottily enforced 50 pound weight limit on bags.
Some divers get around the restrictions by wearing their weight belts, and even their BCDs or BP/Ws, when they board to lighten their bags. Any gear you don't have, or don't want to carry can be rented on the island, although at rather high prices.
Also, get in the boarding line early for your return voyage. The boat fills up fast and seating choices become limited.
Think about upgrading to "Commodore Lounge" for the return trip and getting a guaranteed seat and priority boarding, among other minor perks.
Currents are seldom a problem in the dive park. Avalon is on the lee side of the island, and sheltered from most of the ocean's stronger influences. The one exception is when the off-shore winds from the east kick up, but that's probably not going to happen this time of year.
Yes, take a light!
Peeking between the rocks you'll see lobsters, cleaner shrimp, eels, and all sorts of invertebrates.
As for sighting bigger fish, one just never knows what will swim by. Rays are frequently seen, swimming or digging into the sand with their snouts looking for something to eat. But the real star attractions are the Giant Black Sea Bass. Often seen wandering through the kelp forrest, these guys are very big and slow moving, and incredibly tolerant of divers.
Finally, bring a heavy dose of patience.
In the summer the dive park can get crowded and chaotic, especially since they started renting snorkel gear to non-diving tourists.
With the summer crowds and the hot weather everything takes twice as long as you think it will.
Have a great trip.
K