That is really a unique request and you and your significant other are to be commended for such open communication. Are his wishes legal? The short answer is no. This
non-traditional "burial" sounds very much like a "Viking burial" which has been featured in some movies. Although it looks like a good option there, in reality there are a lot of reasons that it's not allowed by law. One of them is that when the Vikings did it, it was in a huge body of water, like an ocean or major sea, where there were no consequences of anything remaining being washed up on shore or of it contaminating water supplies.
Because he loves his farm property and you want to honor his wishes, here are some alternatives:
* You could have his body cremated and placed in a wooden urn; then place the urn in the boat that will be set on fire.
* His body could be cremated and the ashes scattered on the property. (This, like the previous option, is legal as long as you own the property.)
* Most crematories require that a person's body be placed in or on some type of rigid surface or container in which to be cremated. You may be able to use a wooden or flammable portion of his boat, to place his body on to be cremated with him.
It might be helpful and also give you peace of mind, to call a local funeral director to talk about his wishes and to see if there are other options. A reputable caring one will be glad to talk to you about this.