Andy, I dive with both a computer and a watch. In the event of your computer going mammaries uppermost, then you can use a watch in conjunction with an independent depth gauge and your tables to carry on the dive or at least ascend safely. This has never happened to me ,(yet ), but I tend towards the principal of 'better to have it and not need it than the other way round'.
As far as the various models etc, all I can say is ignore anything that is rated for less than 200m. The ones that say 50m will leak in the shower, 100m ones are ok for snorkelling but little else.
Doubtless, somebody else will be along in a minute with much better information.
Dive safe.