Wet-suit, traps a thin layer of water (which wicks away heat over 20 times quicker than air). You body then heats this layer of water to keep you warm. If the suit is working right and does not have any holes in it, there should be very limited exchange of water during the dive. Disadvantage: Not as warm as a dry-suit. As you go deeper the neoprene insulation layer will compress providing progressively less insulation the deeper you go as the water coincidentally gets colder. You have to buy the thickness of wet-suit suitable for the water you will be diving. This means a 7-9mm for cold water (or thicker) and a 3 mm or rash-guard for that Caribbean vacation). With increased neoprene thickness comes additional buoyancy at the surface and the corresponding increase in lead weights, and of course more air in the BC at depth with the loss of that same buoyancy. Hard to pull on. Advantage: more streamlined than a dry suit and a lot cheaper (you can buy 2 to 7 wet-suits for the price of one dry-suit. Neoprene is durable and comes in a variety of thickness. Wet-suit designs such as farmer john increase warmth by adding layers on the trunk. When the need arises relief is just an Ahhhh away.
Semi-Dry Never dove one of these. Semi-Dry are wet-suits with seals in the wrist and ankles to further limit the amount of water entering the suit. In a perfect fit, your trunk may stay dry (doubtful). Disadvantage: Same as wet-suit plus now "relief" may become a problem as there will be less water to...well flush the suit out. Advantage: same as wet-suit, only maybe a little bit warmer.
Dry-suit With a dry-suit, you are in a bag with only your hands and head sticking out. Seals at the neck and wrist keep water out. Since air conducts heat away from your body much less efficiently than water you stay warmer. You could even wear your work clothes under your dry-suit and do a quickly dive on the way to the office. James Bond wears this on his way to the formal dinner. Disadvantage: cost $700-$4,000 or more with most $2,000-$3000. Dry-suits tend to be more bulky and therefore less streamlined. They usually require a few pounds more weight. You also have to buy undergarments (the loft and weight of the undergarments varies with the water temperature expected) at a cost of $300-$500. Extremely hot on the surface while hiking to the beach from the car. Due to the increased bulk, and the air consumption of the suit, you will use a bit more air. You have to learn how to use a dry-suit and it takes a few dives to master it and that shifting air bubble. The dry-suit boots often require a larger fin pocket so you may have to replace existing fins (a friend wears tennis shoes). Since you are staying dry, you must either wait until you are on shore or boat for relief or install a P-Valve (trust me, they are the way to go). Advantage: One suit for any water temperature, just change the undergarment. Keeps you much warmer than a wet-suit. You can wear dry-gloves exposing only your neck, head (wear a neoprene hood for your neck and head) and face to the water. As you go deeper, you add air to the dry-suit to maintain loft, so there is little to no loss of insulation with depth. Reduced exposure to what ever nasties are in the water. Comes in a variety of materials from durable neoprene (heavy and slow drying) to tri-laminate (light and quick drying). Between dives you peel it off your shoulders if you are hot, the suspenders will hold it up, and relax. Dry-suits make that 2nd and 3d or 4th dive more comfortable. On a dive in WA in 17F weather with a windchill bringing the temp to zero (sound familiar?), we left the hotel wearing our thick undergarment (think child's footie pajamas) So nice before the dive not to have to expose any skin to suit up.
I dive Southern California's 50-60 degree waters. Dove a 7mm wet-suit for years. I now dive dry and I am soooooo much warmer.
Recommendation: Given that your water temperatures will be I expect in the mid 40-s to mid 50s at diving depth, I would say go dry-suit to be truly happy and warm, poorer, but warmer. If you go wet-suit or semi-dry I would suggest thick semi-dry wet-suit (7-9mm) of at least 7mm and most probably 9mm of the farmer john variety.
