What was done for those who died at sea in Ancient Greece since proper treatment of the dead was important and Charon charged for rides?

Ancient Greek burial practices is a really broad topic that I don’t know much about, unfortunately…  Just from poking around on JSTOR and Google Books I found some articles that may be of interest to you and help answer your question (I can’t speak for their accuracy, but you might look through the bibliographies for further sources): “The Sea as a Place of No Return in Ancient Greece” by Astrid Lindenlauf and “Burial Customs, the Afterlife, and the Pollution of Death in Ancient Greece” by FP Retief.