^^^
I use a similar bag.... its just called a dry bag. Designed for kayakers to keep water out but also made by surf companies as wet gear bags. Mines a 60L XCEL bag which holds alot but 100L would probably get boots, wetsuit, harness, pump, bar, fanny rags and more.
http://www.wetsuitoutlet.co.uk/dry-bags-c-38_82.html. If you wanted to keep the boots on the board maybe you could source a body bag.
www.usabodybags.com/
