You don't need a watch. What you DO need, is a good piece of rope, which will probably cost you as much as you're planning to invest in a watch. You can then proceed to make marks on the rope and you know how deep your plate is.
If you insist on a watch, you can buy a cheapo water resistant watch, and time your dives. From the rope you know how deep, from the watch you know how long...
I bought a "Prolog" cheapo watch, water resistant to 100 meters, and it has worked very well since 2006 - I've even changed the battery once without losing water resistance - and I've been down to 30 meters deep with it regularly. I also made good markings on a rope, and the estimated depth has been very accurate when tested with a friends' diving computer.
If you insist on a watch, you can buy a cheapo water resistant watch, and time your dives. From the rope you know how deep, from the watch you know how long...
I bought a "Prolog" cheapo watch, water resistant to 100 meters, and it has worked very well since 2006 - I've even changed the battery once without losing water resistance - and I've been down to 30 meters deep with it regularly. I also made good markings on a rope, and the estimated depth has been very accurate when tested with a friends' diving computer.