You don't need much stuff to windsurf around Britain. Here's what I find useful (in fact a pretty accurate inventory of what I am carrying).
Stuff in the barrel (by row, left to right, top to bottom):
Down jacket, couple of running t-shirts, shorts, tracksuit trousers, microfleece, packtowel, YB tracker (stands for Yellow Brick, as in road), sleeping bag (quite a good down one, packs very small), inflatable mattress, bivvy bag (note, no tent - the rigged sail serves as a shelter), gas stove, cooking pot, cup, coffee filter, coffee, utensils, plasters, tape, soap etc - mostly surplus! Repair bits: Araldite, contact adhesive and patches, stickers - all quite useful. Powerbank - very useful. Cuppa Porridge - easy and filling way to start the day. Snickers - backup supply. Oat cakes. A backup meal or two. The barrel is totally dry even if the board becomes inverted and barrel immersed. Just camping gear in here, plus the best piece of kit of all - the tracker. I only take the lid off on land.
Stuff I carry:
Aquapac rucksac with: ACR ResQlink+ personal locator beacon (on shoulder strap). Drybag (double protection to increase water resistance). Extra layer, random food (often cheese and oatcakes, pictured...flapjack), solar panel from mobilesolarchargers - works well, cables, mains adaptor etc, map case and road atlas, pen, tidal atlas printouts, tide times card, Standard Horizon handheld VHF - still working :) Toothbrush and paste - keep here for ease of access, wallet, waterproof head torch (Coleman from Argos IPX8 and dirt cheap), mosquito / midge net (bought in Scotland, essential kit up there), sun block - refilled a few times now, spare batteries for GPS (in case of on water failure - never happened). Fleecy hat (not shown because I've lost it).
I don't generally open my backpack on the water, but could do if required. Most stuff I consider safety items I keep in the side pockets (radio, head torch, fleecy hat). For quick stops on land it is convenient to have a bit of food, map, means of charging phone in this bag.
Stuff I wear:
Thermals, a thin fleece or (usually) two, excellent Gul Fugitive drysuit. In the pockets (all tied on): Basic GPS with clip to attach to boom, phone in Aquapac bag, knife, Snickers bar x 4 (at start of day, decreasing in number as day proceeds), hiking compass as backup in case of GPS failure or in low vis, Gul Viper boots. Prolimit harness. Gul sunglasses. Sun visor.
Almost all the photos are taken with the excellent and rugged Olympus TG-3 - this is a great camera and has been mistreated daily for nearly 3 months and seems to be happy as on day one.