http://www.randa.org/en/Playing-Golf/Quick-Guide-to-the-Rules/Ball-Unplayable.aspx#ooid=FmeDJqMjpe_ReN--jCRD-5TlVE9FZC0c
If your ball is in a water hazard, the unplayable ball Rule does not apply and you must proceed under the water hazard Rule if taking relief. Elsewhere on the course, if you believe your ball is unplayable, you may under penalty of one stroke:
- play a ball from where the last shot was played, or
- drop a ball any distance behind the point where the ball lay keeping a straight line between the hole, the point where the ball lay and the spot on which the ball is dropped, or
- drop a ball within two club-lengths of where the ball lies not nearer the hole
If your ball is in a bunker you may proceed as above, except that if you are dropping back on a line or within two club-lengths, you must drop in the bunker.
randa.org