Do fertile Rosa rugosa seeds sink when fully ripe and float when not. Johannes
No. They have a spongy mesocarp.
See the section “seed dispersal”.
https://www.researchgate.net/publication/227983324_Rosa_rugosa_Thunb_ex_Murray
They seem to have some mechanism of creating surface tension so they all float.
Iris pseudacorus, also a edge of the water plant, exhibits similar structure. I am happy that I did not take the advice of an old gent who was adamant that all fertile rose seeds sink. Thanks Henry.