Most people see Polynesia as being bad too so it's not a good comparison if you're trying to make Denmark look favorable. Compare it with a good water civ; say, England.
Ah, but there you're going from one extreme to the other. I already said Denmark is no power civ, yet you'd prefer to compare it with the civ that gets the best ship UU in the game (sure, Sea Beggars are great, but not only is SotL a Frigate, and so by default superior to a Privateer, it gets the English UA bonus), and moreover one of the game's strongest overall warmongers (both UUs are not only ranged, but replace the best ranged units of the medieval and Renaissance eras respectively, and to top it off both have good UU bonuses). There's almost the entire game's-worth of power levels between England and Polynesia.
What Denmark is, is a very well-realised theme civ that is fun to play and still strong enough to be competitive more generally than it's given credit for. No, it's not going to beat England in any capacity except, much of the time, early exploration and settlement.
Are those Danish catapults really going to have a higher chance of success than a full charge towards Ships of the Line? I think not. You might seize a capital earlier as the Danes but it's near impossible for a Lizzy player to screw up a water dom game
But seizing early can make the difference. You don't need to pigeonhole yourself to playing a domination game when you have the ability to grab Thebes early enough to transition to a culture game, for instance. Think of the way Attila, say, is pretty terrible for domination outside small pangeas, but he does have the ability to grab a few key cities pretty early, setting himself up to build a more peaceful mid- and late game. Seeing Denmark as simply an inferior, map-dependent domination civ is much like seeing Attila the same way: in any game where Attila doesn't have the opportunity to completely wipe out his rivals in the early game (which is most non-pangea maps, and larger pangeas), he's no use for domination since his units are obsolete, both of them promote into units that can't use the offensive promotions they've earned (battering ram to ranged, horse archer to melee), and his UA gives him no combat bonuses. The solution? If you start on an island, don't play domination with the Huns, focus instead on exploiting early Animal Husbandry and the production bonus.
The Danes are subject to the same risks as everyone else is in early Pangaea warfare with how fragile catapults are.
Generally yes, although there is another exploitable consequence of the UA - they can disembark and embark in the same turn. With enhanced movement (I've mentioned Compass, but don't forget Great Lighthouse and/or the Exploration opener), a unit with Blitz or Logistics can disembark, attack, run out of retaliation range and jump back in the water - ready to disembark and get all that bonus movement again next turn. This can protect catapults and their successors simply by making them hard to catch. Not to mention, if they do take damage, they can pillage without using any of those numerous movement points.
The moral of the story: if you're going to go a-viking, do it in style. And with catapults.
