How should I pack for 70 degree weather in Los Angeles?

70 degrees is about the temperature indoors. So if it's going to be 70 outdoors, then wear what you would be comfortable wearing indoors.

If 70 is the predicted high for the day it means it'll be colder than that the rest of the time, especially at night. The trick to dressing for So Cal weather is to employ layers so you can bundle up in the early morning then take your sweater off when it warms up.

74 with the sun out can feel quite hot. There will be a difference depending on whether you're near the ocean, with a breeze, or inland. Take light clothes, with a jean jacket to throw on if you feel chilly. You may feel fine in shorts and short sleeves during the afternoon, but the lows are in the 50s, so it will be chilly in the evenings and early mornings 

In my opinion, the ocean water around LA isn't warm enough for swimming even in August.  No chance it will be warm enough to swim in the ocean in January.  Regardless, January is not beach time in S. Cal.  

However, 70 degrees will feel very warm to you coming from Ohio this time of year.  Sitting in the sun someplace where any wind is blocked might feel hot.  When you go to places away from the beach (say Hollywood) it will feel very warm. I'd suggest you want to bring a mix of cloths, some summer cloths and some fall weather cloths.