Lycra skin and/or 3-5 millimeter wet suits are recommended. However, your thermal protection should be based on your individual sensitivity and comfort.

Here is a suggested guide:

Month Wet Suit
Jan-March 3mm or 5mm full suit
April-May     3mm full suit
June-Aug No wet suit required
Sept-Oct 1mm - 3mm wet suit
November 3mm - 5mm wet suit
December 3mm - 5mm full wet suit