I've never been to Galveston, but in Southern California when I meet up with some masters swimmers during the summer we swim by buoys or landmarks.

Most of the time we will start at a pier, and swim parallel to the beach. Sometimes we swim to the next pier about 1.5 miles away regrouping a certain buoys, or lifeguard towers. Some people get out an walk back, others may swim back.

If you want a harder workout if you can figure out which way the current is going you can swim against the current.

Another workout we have done is do shorter sprints. Start on the beach, swim out past the surf around a buoy and back to the shore and repeat.

Typically I just go parallel to the shore (distance from shore depends on things like water depth, surf breaks, etc.). The exception is when there are specific buoys (or islands, or opposite shore) to swim to (and a lack of motorboat traffic).

Having a kayak with you is great. Provides a lot more safety, and they can sherpa some bottles for you if it's a longer swim.

For longer salt water swims (sans wetsuit) some bodyglide really helps. You may get chafing on your upper arms, flank, and thighs that never manifests itself in the pool or in fresh water.

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Watch out for the morons fishing off the shore.

My personal experience is that extended swims parallel to the shore is workout enough. Waves make it tough to do any kind of meaningful structure.

An easy warm up for 15 minutes to get used to what the surf and or current/sweep is doing followed by 45 mins of In's and Out's past the break or just about 100m to your buoy.
or
Same warm up followed by "Out" past the surf, swim up the beach for about 200m then swim "In" to the beach and run along the sand back to the start.Repeat until it is beer O'clock.

I wouldn't be swimming. Sounds like you'll be in the middle of an oil slick with that visibility.

;-)