Outdoor pool workouts with no clocks

why not simply count for your rest intervals?  For backstroke,  you'd either need to get a reference from the side of the pool or count strokes.  you can also make up your own sets ie push off the wall hard under water, break out, take 5 strong pulls then flip from back to your breast and swim easy for the rest of the 25.   Do that about 16x25 resting to a count of 5 and you will really feel it :-)   You can also swim your intervals in a similar fashion mixing hard and easy (or "smooth").  Example of a recent set we did was 150s with a rotation ie 25 easy 50 hard 25 easy 50 hard,  rest to a count of 10 or 15 then do 50 hard, 25 easy, 50 hard 25 easy etc.   By changing up the effort and focus during a swim, the pace clock becomes less important and you can count your rest interval.   Also +1 on buying a watch or put your phone in a baggie and use that.

why not simply count for your rest intervals?  For backstroke,  you'd either need to get a reference from the side of the pool or count strokes.  you can also make up your own sets ie push off the wall hard under water, break out, take 5 strong pulls then flip from back to your breast and swim easy for the rest of the 25.   Do that about 16x25 resting to a count of 5 and you will really feel it :-)   You can also swim your intervals in a similar fashion mixing hard and easy (or "smooth").  Example of a recent set we did was 150s with a rotation ie 25 easy 50 hard 25 easy 50 hard,  rest to a count of 10 or 15 then do 50 hard, 25 easy, 50 hard 25 easy etc.   By changing up the effort and focus during a swim, the pace clock becomes less important and you can count your rest interval.   Also +1 on buying a watch or put your phone in a baggie and use that.