I agree with what has been posted so far...
I was thinking, if your mom decides to go with the ocean idea, could you someday take a trip to the Hudson and go out on a boat with some flowers and maybe a letter to your dad and have a little memorial of your own? I guess that is what I would do, just thought I'd put the idea out there.
Best of luck, I hope that you and your mom are able to agree on something. Take care!
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From the movie The Hours: "If I were thinking clearly, Leonard, I would tell you that I wrestle alone in the dark, in the deep dark, and that only I can know. Only I can understand my condition. You live with the threat, you tell me you live with the threat of my extinction. Leonard, I live with it too."