Where do you want to go?
If you're looking for the beachy appeal of the Caribbean or for the rugged outdoors of Alaska, the answer is easy (ocean). But if you're looking at Europe, or even Asia, you want to evaluate where you've already been, and what's at the top of your bucket list. Ocean cruises in Europe, for example, focus primarily on the Baltic or the Mediterranean, while river cruises make their way through the interior of the continent, and offer a wider variety of choices of destinations.
Does demographic matter to you?
While ocean-going cruise ships vary from ones that cater to active types of all ages, to those with lots of young families with kids, those with plenty of empty nester couples, and those filled primarily with retirees, river cruises have historically skewed toward the higher age groups. While that age has dropped over the last five years, you're still less likely to find 20-somethings and 30-somethings on river cruises.
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Do you need a lot of choices for how to see the ports?
While most ocean-going ships offer anywhere from a handful to more than a dozen shore excursions to choose from in every port, most river lines have a walking tour (with a miked-up guide who speaks into an earpiece) and then, sometimes, a few optional add-ons, like winery tours or biking expeditions.
Do you need a lot of dining and nightlife options?
Because river cruise ships are constrained by the width of the rivers and the heights of the bridges, these ships top out at around 200 passengers. The result: you're going to find one or two restaurants, as opposed to as many as 18 on the biggest ocean cruise ships. Ditto for bars and lounges: there will only be one or two, as opposed to a dozen with different themes, drink lists, and styles. In addition, while the line might bring onboard local entertainment to amuse passengers, you're not going to find big stage shows or crowded dance floors on these smaller, social, but quieter cruises.