Can you Swim without adding pool size

You certainly can swim, but the detected distance will be wrong.

You could try using the Open Water Swimming activity profile, where in case of low GPS signal, the distance is estimated from the stroke cadence, but you may need to calibrate it first in a few true longer OW swims.

Thanks, just want a rough idea as to distance in  holiday pool

The watch counts the turns when you use the Pool Swimming activity. So either set the right pool length, or (if it is less than what the watch allows), then set the double of it, and once the activity is synced, edit it, and halve the number of length.

Thank you