The Seafarers are overall very good older classic fiberglass boats. Like many boats of their era, the decks on some models are balsa core. Soft decks are obviously indicative of water intrusion through an area with a weak seal. Most likely a deck fitting, or some modification by a previous owner has caused the problem.
In my opinion, an otherwise good boat should not be parted out just because of this problem. Repair is possible and fairly common. I've seen several boats other than Seafarers with the same problem and were repaired by their owners. Once non-skid paid is applies the repair shouldn't be detected.
I do not think the asking price of $15k is that great of a deal with or without the soft deck issue. With the issue I think the price is entirely unrealistic. Others have suggested that the boat should be surveyed and professionally appraised. I could not agree more.
I certainly hope the S34 is saved. They are wonderful boats with a very loyal following. I have owned a S29 for over 12 years and have been very involved with other Seafarer owners for just as long.
If you're interested in specific advice on the S34 I suggest joining the Seafarer Yacht Group on Facebook. Currently, we have over 150 owners/members, and to my knowledge this is the largest Seafarer member group in existence. There is also a good forum for Seafarer owners on this web site. Here's the link to the Facebook group.
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I look forward to hearing more about the restoration.
sv Roma - Seafarer 29