Perhaps multiple bottles. I was the one (oldest male) selected to send my Aunt on her way. The ashes were actually sort of heavy and would have needed a 2 liter bottle as someone else suggested. Not sure a bottle full of ashes would have floated.
We didn't use a bottle(s), but did send her ashes out to sea. The funeral home had my Aunt packed in a plastic bag. We took her with us to Ocean City MD. As nobody wanted to do this by boat, it took a while to find a spot along the beach that didn't have people. I waded into the surf and tried dumping her out. Some of her ashes came out and the wind
blew them all over me. Try as I might, I could not get all of her out of the bag.
Maybe it was divine intervention, but as I slowly walked farther out, a breaker hit me hard and when the water
receded, I was soaked, but the ashes were gone and the bag was completely empty. Mission accomplished. I was still laughing about it when I returned to my family
at the top of the dune path (my Mom was in a wheel
chair). Of course they were all still crying, so I had to bite my lip to get the smile off my face. But into the Atlantic she went. She arrived in America by boat, it was fitting she return to the waters she once crossed.