Well, I can tell you right off the bat that mareanie and binacle are marine pokémon. They won’t exactly die if you try to keep them in fresh water (that is, rivers or presumably the canals you’re thinking of), but they won’t exactly thrive either. So because you have a choice in the matter, I would suggest avoiding those.

Of the remaining two, shellos (while also technically a marine pokémon) does excellently in rivers and canals—the colder and cleaner, the better. This pokémon is native to colder regions such as Sinnoh, so it adores fresh, flowing waters, especially those fed by snowmelt. Meanwhile, lotad is a far gentler pokémon that is often more content with drifting … which means it runs a risk of being caught up in a river’s ebb and flow.

Additionally, lotad, being part-grass, don’t fare as well in polluted waters, so if you’re planning on letting your lotad out in the waters near your home, you may wish to keep it away from the canals unless your city’s canals are particularly well-maintained (and perhaps not even then). Shellos, on the other hand, are far hardier and can withstand polluted water (although, yes, it prefers unpolluted, as noted above).

That isn’t to say lotad isn’t a worthy partner. Many trainers prefer lotad for their easygoing dispositions, which make them rather easy to care for on land. In fact, so long as you have calm water and food, a lotad will typically be perfectly content. Some people even feel that just looking at a lotad floating happily in the water is calm and soothing to them, so lotad tend to be popular choices for homes near slow-running rivers or at least backyard ponds.

By contrast, shellos do actually require some level of basic care beyond food and water, including daily affection, grooming (simply wipe a wet towel across their bodies to remove excess mucus or debris), and exercise, and it’s not unusual for curious shellos to escape any enclosure you put them in (which means you may need to keep your poké ball on hand to prevent a curious shellos from wandering too far from home).

Finally, there are the evolutions to consider. Gastrodon is a beautiful pokémon often admired by collectors due to its bright colors, and for non-collectors, it’s even hardier than its preevolution and can thrive in practically any sort of body of water. On the other hand, it doesn’t fare too well out of the water, and it requires constant hydration. Meanwhile, lotad becomes progressively more amphibious as it climbs its evolutionary ladder, but if you were hoping for something peaceful and lethargic, you’ll need to stop at lombre and avoid giving your pokémon a water stone if at all possible. Ludicolo is a high-energy pokémon that can be quite a handful for trainers who aren’t also energetic.

In short, get a shellos if you want a hardy, energetic pokémon and don’t mind keeping it in water at all times after it evolves. Get a lotad if you want peace, a pokémon that’s easy to care for, or a dance partner later on down the line. 

Best of luck!