Both parts need 3 bottom layer. The nose is then a closed hollow part and should be absolutely water-tight.
If the PLA is transparent enough, you can see the water inside shining through it with a flashlight. Or you weigh it before and after.
If the worst comes to the worst, it is then the life buoy ;-)
I glued the two parts together only with double sided tape. So I can exchange the nose if damaged - or take one with another color.