Yarn: worsted weight, any fibre (I used Willow Yarns Fawn Worsted)
Approx 50 yards or half a 50 gram ball
Hook: 5mm (H/8)
Size unstretched: 7 inches deep, 7 inches circumference, shoulder strap approx 33 inches.
This is very stretchy and will fit up to a 40 ounce bottle. The depth of the bag and the length of the strap allow for stretch when holding a full bottle.

Abbreviations:
(I have used US terms – UK in parentheses)
ch chain
dc double crochet (UK treble)
dtr double treble crochet – wrap yarn three times around hook [yarnover, pull through two loops] four times (UK triple treble)
ss slipstitch (when working in rounds, ss means to slipstitch to the third chain from the join of the round below)
sp space
rnd round
Starting chains at beginning of round – the first three (or five on the strap) represent the first stitch plus any chains for spaces

Edited to add: You are always working into the top of the stitch in the row below, not the chain spaces.

Method:
Make a slipknot, ch 5, join by slipstitching to first chain. Work over your yarn tail for the first round.

Rnd 1 – ch4, (dc1, ch1) 9 times, ss to join (10 dcs and 10 ch-1 spaces)

Rnd 2 – ch5, (dc1, ch2) 9 times, ss to join (10 dcs and 10 ch-2 spaces)

Rnd 3 – ch6, (dc1, ch3) 9 times, ss to join (10 dcs and 10 ch-3 spaces)

Repeat rnd 3 for ten more rounds (13 rounds total from beginning)

Next rnd – ch 3, dc 3 into space, (dc 1 into top of dc below, dc 3 into space) 9 times, ss to join
Do not fasten off.

Strap:
(Ch 5, dtr into next two stitches, turn) 32 times (or to the length you want, bearing in mind it may need to be longer for larger people, shorter for small people, and allowing for stretch when holding a full water bottle, and make it an even number)

Lay bag and strap on flat surface so nothing is twisted and you can identify the three stitches that are directly opposite the place the strap is already attached.

Holding the loose end of the strap against the inside edge of the bag, working loop on the right, sc through the stitch of strap and the stitch on the bag three times to join the two pieces together.

Ch 1, fasten off and weave in ends.