Weddings Were Land Based, In Churches, Very Purpose Driven
A royal wedding was a very big deal because the legitimacy of the heirs depended upon the legitimacy of the marriage. They would be hosted in a large religious center with as much fanfare as possible because raucous festivities and hoopla were how they advertised to the public, surrounding nations, and most importantly any would be usurper or ne'er do wells that the marriage was legit. In fact in very old times, advertising every aspect of the marriage as legit was so intense that often time religious officials would witness the... ahem.... consumation so that it could be testified that the entire process was complete and verified by a higher power as totally 100% genuine.
Obviously by the 1800's royal wedding traditions were just based on a lot of that stuff from the medieval era, but it was still a fairly embedded tradition. In Victorian times image was everything, and putting on a good wedding and accompanying display of wealth was required to ensure one's subordinates were adequately shown up and kept in their place. Again, a big part of this was crazy elaborate ceremonies and no expense spared celebrations that you ideally wanted as many people as possible to see. On the sea this is pretty tricky. First, even up until the 1900's sea travel was pretty dangerous. Packing all of your dignitaries, functionaries, and the majority of the royal family onto a few boats on the treacherous seas was basically begging for your countries government and high society to be beheaded. You are vulnerable to storms, icebergs, and enemy attack, even in a big fleet with military escorts it would only require one enemy artilleryman to get lucky and kill somebody incredibly important and irreplaceable. A storm could simply sink and drown them all.
But Lets Ignore all of That
I'm assuming you meant 18th century as in the 1800's, the actual 18th century (1700's) nothing like any ship big enough for a full blown royal wedding existed, plus most large vessels of the 1700's were war ships and pretty horrible to live/spend any amount of time on. In the 1800's the RMS Oceanic was the largest passenger vessel of the era. It could carry 1,700 passengers, this is juuuuust big enough for a big fancy rich wedding to occur on. It's a more modern design and thus both much safer and much more comfortable than anything prior to it. Deck that baby out in royal finery and pomp and fill it full of slightly inbred snooty aristocrats and boom. Got yerself a plausible royal sea wedding.