Wetsuits come in all types of flavors. Generally, you start with the thickness of the suit, in mm. Often, they are a combination. Here in Socal, the water ranges from mid (sometimes) high 60's in summer to mid 50's in winter.
I surf in the winter in a full suit that is 4/3 (4 mil chest, 3 in the arms). Very cozy. I rarely use that suit for sailing in winter; mostly I sail in a full suit that is 3/2.
You can also buy Spring suits, which are short sleeve and legs. They are usually 3/2, sometimes thinner. I wear a Spring in the summer.
Dive suits tend to be too restrictive to sail or surf in.
I always wear wetsuit booties.
What you buy depends upon your local conditions. Remember that a wetsuit works by allowing some water to get in and warm that water. You can also consider dry suits, breeze suits, etc.
Hope this gets you started.
Sheldon