I've heard August in Holmes Harbor can be good as the fish linger waiting to run up the river. I'm sure there are fish around now too however. I've not fished Penn cove, but I'd think there is a lot of food for the Sea Run Cutthroat near the mussel beds.
It's almost never necessary to fish deep for sea-run cutthroat, nor is it necessary to cast long distances. Cutthroat usually feed close to the beach. In fact the old-time cutthtroat fishermen used to say (when fishing from a boat) "if you can't see the bottom you're fishing too deep". Leland Miyawaki says that he rarely casts at more than a 45 degree angle from the beach. I never use anything heavier than an intermediate-sink line and usually a floater. Surface flies (Leland's Beach Popper, Steve Raymond's Cutthroat Candy) are an effective and exciting way to fish for cutthroat (and coho).
Caveman, the answers are: right now, and bait fish imitation.
Go down to a rocky beach, especially around a point, and wade, looking for bait fish, noticing their color, and size. Go to a fly shop that caters to salt water FF, and talk to them about fly selection, pick up what they suggest, but also anything that resembles what you observed. It keeps getting better through July.
I mainly fish cutthroats year round and have figured out a few things. For now, as summer continues, searuns move out deep up to 20 feet deep (which are only found by boat guys.) during the day and in close at night. At least that is what I have noticed. Also keep in mind once you think you have them figured out, you don't. Try damn near everything with a few hands down rules. (see search and read RStevens posts) he does a good job at breaking it down. But cover the beach as much as possible, either by foot or by boat (I prefer boats because private beaches are yours for the taking.)