Individual tolerance to cold makes a huge difference, as does the depths/bottom times you have in mind. For recreational diving, the majority of people I see are in wetsuits even up north. Heck, there's an instructor in Monterey who wears a 7mm wetsuit without hood or gloves and does fine. I hear San Diego is colder, but if you're primarily looking to dive in the Channel Islands, where the temps are high 50s to mid 60s, then a semidry should be fine. If you plan on diving even colder waters in the future, maybe it would be best to just invest in a drysuit now, but otherwise spending $400 on a good semidry makes more sense to me than going for a drysuit+undergarments. Aqualung Solafx and the Hollis Neotek are both great choices. Fourth Element's Proteus is the warmest semidry I've tried, but it runs $500 which is getting closer to a cheap drysuit.