Could anyone recommend a small ruggedised back pack to store a fin (30cm or less), small water bottle, banana etc ?. Is doesn't have to be water proof. Just salt water resistant, small, and tough with no metal zips/buckles or sharp bits that could slash a sail. Built ruggedised enough so that fin doesn't cut a hole in the backpack and fall out during the session (I'm aiming not to carry the fin cover). Marine backpacks online look well made but are too big (25L). Any tips appreciated 
. J
Look at camelbaks (not necessarily the brand) and similar.
They have water bladder inbuilt, generally have separate backpack pouches. possibly look at tactical version as the camelbak brand(which I use) have metal zips which only last a couple of years before crumbling to dust.
Get unit with waist straps and a strap across chest to hold shoulder straps in place.
Check how bottom of shoulder strap is attached as this is a failure point. The waist straps are better of they are connected to the back pack with a strap with Velcro through a loop rather than hard stitched as this will eventually rip apart with a hard enough impact. the Velcro will come undone but can easily be put back together.
I followed Mark Jordan's lead and had a small camelback type attached to my impact vest. The front pouch can store emergency stuff (knife / line / whistle / energy bars) whilst the camelback holds the bladder of water. Whilst I'm not a huge fan of the Forward Wip vest due to how it rides up on my waist, having the pockets are super handy.
I've got an Ion Booster X vest that has both a small zippered front pocket and a large rear pocket for a hydration pouch. This back pocket would comfortably fit a fin or two. There is even a fixture point inside the pouch to tie things in. What I like about the Booster X is that there is a waist strap that I can clip through my seat harness spreader bar and the vest then cannot ride up.
I ended up buying a Mountain Designs Flow Plug 6 from my local Anaconda store (they only had the 1 model available). I also visited Kathmandu, which had lots of different models on their web site, but they also only had the 1 model in the store I visited. But, the stores allowed me to try the bags out for size. I took 4 of my fins in to the stores to test for fitment, and raised a few eye brows in the process ("What is that man doing ?") 
It can fit my 25 Tribal weedspeed, a Tectonics 31 Matrix pointer and my MUF 18 Delta. My Tribal 29 Weedspeed is too big unfortunately. Could probably have it sticking out, but it may fall out ?. The bag material is a very thin (tent ?) material. It would be good if it was more ruggedised. This will do for now until something better comes along. Thanks all, and to my team mates for the tips
Regarding bladder, I will remove the tube and block it off at the base with a stopper and use the bladder for ballast. The water inside it can get as black as it likes.
