: why not put the bottles in a large cheap stuff sack, garbage bag, etc. just
: to contain them in one part of the boat?

A mesh bag won't hold any water, but it holds bottles really nicely. Should you get water in the boat the mesh bag drains and dries quickly, and you can loop strings through the mesh to give thousands of possible tie downs. After a paddle you cna hose down your float bottles, bag and all, and hang it on a hook to dry.

Look at a sporting goods store for the mesh ags used for carrying basketballs and soccer balls, or you can find the polyester mesh fabric at a fabric store and make your own bags. If the fabric is 60 inches wide, then by buying just 1 yard you get a piece which is 36 inches by 60. Depending on how you fold this in half and you'll be able to make a bag which is either 30 by 36, (with the fold at the bottom of the bag) or 60 by 18 with the fold being one long side of the bag.) Sew two sides and leave one end open. Since the fabric is full of holes, you can loosely "sew" this without even using a needle by just passing a string through the holes in the mesh and knotting it at intervals. I think the 60 by 18 size would be better suited to kayak use.

Of course, you can easily trim the fabric to make a narrower bag, or TWO tapered bags to suit your kayak.
If you fold the 60 x 36 piece into a 60 by 18 rectangle, and then cut that diagonally, you can sew up two triangular bags which taper from a point to about 18 inches wide (when laying flat) at the big end.

At the open end you can weave a drawstring through the mesh, fill the bag with your bottles, and pull the drawstring tight to close the top of the bag.

Hancock Fabrics has mesh materials at:
http://www.hancockfabrics.com/shopping/product/detailmain.jsp?itemID=10163&itemType=PRODUCT&iMainCat=5&iSubCat=199&iProductID=10163

Other fabric stores should have this, or similar fabrics, too. Look for sales, remnants and ugly colors. When there is not a lot of cloth left on the bolt the stores may pull the bolts from the regular spot on the shelves, and move them to a discount aisle, and sell what is left for less, particularly if the color is drab and not selling well at the regular price.

No reason to just use these bags for flotation. Smaller ones are great for holding gear so it does not rattle around, and deck bags made of mesh, wrapped around the bungies, can be handy, too.

Hope this helps.

PGJ