I don't think a lot of people do map runs, either out of fear or misunderstanding of the dungeon' mechanics. They are pretty fast to do, and can be a good night with friends. Additionally, the chances are very real that you can strike it big! So, here's a primer for the canals.
#Prepping
-Throughout the week, folks can gather treasure maps. You get one allowance per day, and each character can only be in possession of one type of map at a time (although I think there is no limit to maps that can be placed in a company chest). For this present era, you want to do relevant content, and so Stormblood maps are the way to go. The easiest stormblood map to routinely gather is from Palladium Sand nodes in the Azim Steppe (very high gazelleskin spawns). Save em for when you and a few mates get together, then do em all at once!
#Finding your map
-There exists several sites online that detail all the possible spawn points for maps. Alternatively, you can find it yourself. Look for geographic features that are similar to your map. The stormblood maps are pretty easy to locate compared to the HW, but it can still be challenging! Once you dig up the chest you'll have to fight enemies around it. When you open it, you get your first RNG of the night: finding a portal, or not. If you did, onto the canals! If not, better luck next week.
#Party Comp
-Although the maps recommend eight people, really they should read as "up to 8 people". You can do these maps pretty safely with four people, and with a strong group can even do it with three. As such, this becomes a pretty easy thing to do even with a small group! Treat it like a standard dungeon (1 tank, 1 healer, rest DPS) and you should be fine!
#Fights
-The trash are pretty straight forward, but even they have some variability. Keep an eye out for the these special fights...
-Fish spawn: A fish person will spawn and run around frantically on the map. He will only be present for a limited amount of time, kill before he runs away for more loot!
-Onion Knights: Five enemies will spawn with different culinary based names. They will have numbers above their head and run around like the fish person. Kill them in order (1 through 5) to get a guaranteed drop that's usually a vase.
-Minotaur Elephant: This works just like a mini boss. Just make sure he's facing away from the group so that he can't cleave everyone, otherwise just ignore he's there.
#Post fight... which door?
-You've cleared a room, looted the chest, and now you are moving (hopefully) on to the next room. Sometimes if you're lucky, the right door will show itself (rare chance that I've only ever seen on the first two levels). Most of the time, you'll have to pick. But, the choice isn't as obvious as it seems.
-It actually doesn't matter which door you pick, because the RNG happens when you pick a door (Ie it's not a 50/50 shot). Although I don't know the percentages, the doors work like this. When you pick a door, one of four things happens: you picked the right door, you picked the right door but it closes first, you pick the right door but it closes and nearly kicks you out (rainbow), and you pick the wrong door. The chances of all of them but "simple right door" seem to increase as you progress in the dungeon. The chance from my personal experience of making it to floor 7 is roughly 10-20%.
-Each level is really a "tier", with the most notable increase in loot after level 4. If you make it to level 7, you'll get a non-trivial amount of gil from the chest.
#Other special things
-Sometimes as you progress you'll get a green map. These are unsellable and take you to a hidden area underwater, with greater loot tables but lower chances of progression. The best treasure can be found in these, and your drop rate is high enough that you'll probably see one if you do a group's worth of maps.
-Sometimes your chest will have a lure on it. This is basically a game of high/low, where the game draws a card and you have to guess if the next card will be higher or lower. Guess right and you double your chest yields. Guess wrong, and you lose everything. You can do this seven times, and I don't know what happens if you get all seven (I usually tap out at three).
Anyway... go out there and gamble! I feel like maps in general are an under utilized aspect of FFXIV, and they can be a ton of fun!