I have used salmon skin for backing bows as it is very thin (therefore light) and tough. I usually colour it by making a tanning solution from oak bark.
get a pile of oak bark roughly chopped cover with water and simmer for a few hours let it sit for 24 hours and strain off the juice. Put the fleshed and descaled fish skins in this solution and give them a stir every so often so all parts of the skin are exposed to the solution. It'll take about 4 or 5 days to tan them. I then rinse them out and roll them up and store them in the freezer 'til I need them. If you don't want to freeze them just scrape, scale and wash them well and nail them to a board until the dry and as long as they don't get wet they'll last indefinately.
I have made a bowstring from the rawhide too but haven't used it so I don't know it's durability.
I have also tried to make leather with limited success, it turned out rather stiff and crackly but worked ok.