When wading in rivers or shallow water at the coast, you cannot work from a rucksack, since there is nowhere to place it close to hand. To save time repeatedly returning to the bank or a raised rock on the shore, to pick up a filter, lens or another memory card, I stash as much as possible in a fishing vest (shorter than a photo vest and so the bottom pockets cannot get submerged). Larger items, such as a flash or another lens are carried in a waist pack with the top flap opening away from the waist, which can be swung round to the back when actually shooting. In deeper water, I make sure that the tripod is firmly wedged into a soft substrate before I connect a camera - especially when using an expensive long lens!