Are the beaches in Rosarito Mexico clean enough for swimming?

I'm quite familiar with Ensenada (considering purchasing a holiday residence there), which is 50 miles to the south of Rosarito. I assume you're asking because of the electricity generator and the transport docks in the northern part of Rosarito, so I'll scope this answer down to south of the fishing pier.

If you'll accept that, then the water quality is about as pristine as you'll find on the west coast with the possible exception of Cabo San Lucas at the tip.

What are the water conditions like there?

If 'water conditions' means clean, then it's fine. If you mean surfing, I don't surf but never saw that many (for obvious reasons). Those that were out surfing invariably wore wet suits.

The water temperature is not that great for me, but it's a matter of opinion. Anyway... it's not the temperature of the water that will bother you so much as the jellyfish and sting rays. Plus sharks come in to feed on the fish. Also, the rocks are really sharp, both above and beneath the water and there's a danger of being pulled down and cutting yourself.

Summary: For me, western Baja is more about the climate, the nature, fishing, and health benefits than swimming. I would not associate that region with recreational swimming because you can do much better further north.

Adding because of commentary...

If you make an exploratory trip to the region for any length of time, I recommend packing a Delorme Communicator. I can't recommend this highly enough for travelling around Baha, especially on the Sea of Cortez side.