About 30 nautical miles in a sixteen foot Savage aluminium dinghy. Currumbene Creek to East Reef, Jervis Bay. Did a lot of fishing in my teens from my dad's boat, but not a long way off shore - smallish craft and only a 20hp outboard, so took a long time to get anywhere.
Furthest I've ever been in a big boat is 14 miles, Sydney to Parramatta on the RiverCat ferry.
Not far: England to Norway or Sweden. Different routes but roughly the same ferry distances, about 400 nautical miles.
Actually the return leg of the Newcastle-Bergen ferry was quite a bit further, by a very attractive afternoon cruise down the Norwegian coast to Stavanger before turning South-West back over-night across the North Sea. The one-way trip was about 24 hours; the complete round took three days and was for a time offered as a miniature cruise off-season.
Sadly there are now no public car-ferries from the UK to any Scandinavian country, not even the nearest, Denmark.
You can book your car and occupants onto a freight ferry to Sweden, but these are really to carry laden articulated-lorry trailers (the tractor unit and its driver stay in their own country). They can carry up to 12 passengers, and though the accommodation is comfortable it lacks the bars, shop, fancy restaurants and other public-ferry accoutrements (perhaps a blessing!), and the ship is a lot slower than the public ones. Much more importantly though, it's something of a privilege, not a guaranteed service despite booking well in advance. It's also a good idea to know your way around vast freight ports with no tourist accommodation, and obey their strict safety rules about where you park and wait.
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