I was not going to respond to your OP until you later added that,
"The pack only needs to be waterproof against rain, storm, snow etc... no full submersion ..."
Given that you are not looking for a drysack-type piece of gear such as those used on (and in, if the boat flips) the water, please note (oz. are ounces per square yard):
Rockywoods has a 3.8 oz. 200 denier coated ripstop nylon in a sand color and intended for military use that might work for you:
McHale Packs has a dark gray 4.4 oz 210 denier coated riptsop nylon with a diamond Dyneema grid: http://www.mchalepacks.com/ultralight/detail/fabric_colors.htm
It is not listed for sale on the site, but Dan was willing to sell me a couple yards.
The coating is of recent vintage, and much better IMO than coatings on other spectra and Dyneema gridstops I have purchased before.
I've ordered and looked at both of the above, and they are both very waterproof; but without an HH tester, I could not tell you which is more WP. Dan may also have some 3.5 oz gridstop, but I don't know if that has the newer coating.
Quest Outfitters had some double-coated material in the 3-4 oz. range also, but ran out of it and the site states there is no expectation of more.
I was going to use somewhat heavier 4.9 oz. Xpac VX07, but once I got sewing with it, became concerned with the stretching of the material at the stitch holes, much the same issue as with the mylar film in Cuben. It also began to wrinkle and pucker into odd shapes when tensioned as part of a backpanel, and looked dreadful at one point where I had to remove some stitching. A recent post on this site showing delamination of some lighter Xpac added to my concern. There is a heavier VX21 Xpac which runs over 6 oz.; but I felt mission creep was beginning to occur, and drew the line.
I also looked at 2 varieties (Warmlite and Seattle Fabrics) of medium green sil-coated 1.9 oz. ripstop nylon that weighed about 2.5 oz with the coating, but they were not as waterproof as any of the above mentioned materials. The 1.9 oz nylon is around 70 denier, also; so is not as strong and durable as the 200-210 denier material. I will use it only for pockets inside the pack. Have used the Warmlite for several years for stuff bags, and it holds up very well.
So I will use the sand Rockywoods material for most of the pack, probably with some of the gray Dyneems grid material from Dan McHale for a bathtub pack bottom with a light coated liner inside it. Am OK with 1.4 oz. Thru-Hiker silnylon even for tent floors, but want something tougher for a pack, as they can get a lot of abuse. Since the T-H silnylon has a very high HH and weighs under 1.4 oz., I will probably use it to line the pack bottom, and probably coat one side of it with a very thin coating of GE sil-glue so it doesn't bunch.
Rockywoods also has some heat-sealable coated nylons in the same weight range that might be of interest for making waterproof seams. I'm content to use the Seamgrip for seam sealing - have found it very reliable.
Hope this is helpful.