I keep hearing about the fog along the coast between LAX and SFO. As we'll be doing the drive in late April I'm wondering if we will miss any of the view because of fog.

Also, we will have just one day to visit San Diego and can't decide on whether to visit the Zoo (we are Aussies and don't want to see koalas), sea world or the Queen Mary.

San Diego Sea World wins if you like whales. A lot a people don't know but Seaworld has a very nice stall of Clysdall Horses with Dalamations and a Beer garten too.
Queen Mary is in Longbeach near LA.

There's an Aquarium in Long Beach near the Queen Mary. I hear it's impressive, though I haven't been there. I did enjoy the Queen Mary.

The San Diego Zoo has a lot of things other than Koalas. Currently, our third Giant Panda baby is the big attraction. You can easily bypass the Koalas. You could also visit one or two of the museums in Balboa Park. They're a short stroll from the Zoo.

There's more to Sea World than Shamu, the Killer Whale, including the famous Budweiser Clydesdales.

On the issue of fog - summer is the foggiest time on the CA coast. This is due to the inversion layer which is created when hot inland air meets cool coastal air. The general pattern is that the fog rolls in during the evenning, stays overnight and then burns off by mid-day. Some days it doesn't burn off at all and some days there isn't much if any fog to speak of. Most of the time the fog is relatively high, but sometimes it comes down to ground level. Spring and fall are generally the clearest times of the year. Winter time there generally isn't much fog, but you've got clouds and storms to deal with.

So the bottom line is that April is a very nice time to visit. There probably wont be much rain, fog will be minimal or nonexistent, everything should be green from recent rains, and the wildflowers will be in bloom.

As for SD, I've been twice and not been to the zoo or Seaworld.