For the 5% small-stuff adjustment, really depends on what the issues are. If this is a pretty old boat in moderate condition and the defects were clearly visible from original viewing, then the seller might push-back on accepting a credit. If buyer has priced the credit as a replacement-value, seller might push-back.
It really depends on how reasonable the buyer is being. But to answer the OPs question, yes, a
survey credit is really common. But a lot of sales fall apart due to unreasonable buyers or sellers. Especially in this market, buyer doesn't have great leverage given he's now out-of-pocket for survey and haul.
I'll echo other's
advice to not sweat the small stuff. I have owned my boat for 23-years. I received a significant credit for
blisters. But I can honestly say I do not remember the final selling
price. What seems like a big deal at the front of the deal fades as
ownership takes over.
Peter