Why is the pool harder than the ocean??

That's a really good question, and I can only speculate on the reasons.  I grew up swimming in the ocean (Long Beach, California, north of you) and didn't swim in a pool until I joined the high school swim team, back in 1977.

My thoughts:

1.  Saltwater would make you more buoyant than pool water.

2.  In the ocean, there are no walls, so you are not using your muscles in the way you do when you push off the wall at each end.

3.  If you haven't done flip turns in a long time and you were doing those in the pool, that could have worn you out.

4.  If you weren't doing flip turns, you are still breaking up your swim at each wall by doing something you weren't used to:  turning.

4.  You are relaxed in the ocean, because that is "home" to you, and you adapted to that environment.  The pool environment was way back in your past, so you weren't as comfortable there.

Forumites, can you think of anything I missed?

You doing flip turns?  Try open ones.  I spend half my warm up doing open turns before going to flip turns.  When you flip turn, you disrupt your breathing pattern, as you have to hold your breath for the time it takes to lead into the wall, flip, push off, and break out.  It is very, very different than open water.

My breathing in and out of turns can be inefficient so I will become winded more quickly swimming in pools. But then my first open water swims each year result in sore shoulders more quickly.

I find open water a easier than the pool