Krakonos, a Pirate-Themed Draft Cube

This is a cool concept. Pirates have plenty of uncharted territory to explore in Magic. I also really like the idea of non-human pirates, and that color shard is perfect for it.

Plunder is a good idea and makes a lot of sense, but it's a mechanic within a mechanic, which I don't think ever happens. Instead, you might just make a lot of creatures with that ability and not keyword it. Unfortunately that means you would need another mechanic for that colour combo, but isn't treasure good enough, or are you using treasure across all the factions? Voyage has the same problem. I suggest you try to invent something that doesn't involve suspend. You might consider using something similar to adventures from Eldraine, or just use adventure itself. Cajun just recently finished the template for it on the forums. If the idea is for a bunch of ships and creatures to go on a journey together and then come back and do something, that pretty much describes adventures. The only difference is that vehicles aren't involved, and it's on an individual card basis. There's definitely some potential here, though. 

As for the other shards that still need mechanics, I have a few ideas. Pirates and sailors are very superstitious and full of tales of ghosts and monsters and other things lurking on the high seas. I suggest you lean into that and utilize some type of fear mechanic for the ghost ships. Don't use fear, obviously, as its considered obsolete, but maybe it could be something similar like Terror (This creature can't be blocked by Pirates). It's very simple and has good synergy with a lot of other mechanics, and it's also potentially very powerful. Another version could be something like Terror (Whenever this creature attacks, return target creature defending player controls to its owner's hand). That communicates the idea of Pirates running in fear as a ghost approaches. It's very strong so you'd have to use this sparingly, or put some kind of limitation on it (based on creature power, give it an activation of some kind, whatever). For the merfolk, I'm less confident, but merfolk, from ancient times at least, have almost always been nefarious. Some sort of mechanic involving forced attacks on you and then granting you some kind of benefit would make a lot of flavor sense. For instance, Lure COST (Target creature must attack you or a planeswalker you control this turn if able. Destroy it at the end of combat. Activate this ability only at the beginning of combat during an opponent's turn.) There's probably a better way to word this, and possibly a cleaner execution, but the basic idea is an irresistible siren or merfolk luring unwary sailors to their doom. Merfolk tend to be a small tribe that relies on numbers rather than size and feature a lot of tokens or counters, so you might be able to use that to your advantage too. Drown COST (Target tapped creature gets -1/-1 for each Merfolk you control). I dunno, just brainstorming over here...

For rarities, You let the card complexity tell you what rarity they are. Commons get basic stuff done, but don't usually feature anything new, complicated, or splashy. Uncommons are also mainly functional, but can be quite a bit more complex and have multiple lines of text. Rares help you win games. Mythics, as Mark Rosewater puts it, just win. You want every Mythic to feel unique and memorable. If you just start creating a bunch of cards, though, you're going to make tons of cool and powerful designs and end up with just piles of rares and mythic rares. In order to keep things under control, I suggest restricting yourself to a card skeleton and filling it in with the rarities and card types you need. I highly recommended going through these articles from the Nuts and Bolts series that describe how to create a standard Magic set in detail. The design skeleton will help you figure out what you need and the numbers of each card type at each rarity. Then you can begin your designs and it will really help you streamline the process. If you have too many cool designs that you want in the set but no space for them (as I often do), just put them aside for now and create a second set in the same block. Then you have two or more awesome sets to draft from, and they will naturally work together because they began as one set!     

On another note... You are industrious aren't you! How many sets are you working on right now exactly!?

First off, ignore rarities if you're making a cube - cubes are supposed to generally consist of cards of equal power, since all of the cards are singletons. Since the primary purpose of rarities is to control how often a card shows up when cracking open packs, it's irrelevant for cubing.

In any case, I have to disagree with fluffydeathbringer - Voyage is a fantastic mechanic, but maybe not for the reason you intended. It's fantastic because it acts as temporary creature removal. In Green. (Notice how it does not specify creatures you control). However, Scout Ship is pretty overcosted for what it is - I'd mark it down to maybe , personally?

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For your Esper Ghosts and Grixis Merfolk factions... what kind of playstyle do you want them to have? It's kinda pointless to try to come up with solid mechanics if you aren't sure what they'll be used for.

I mean if you wanna add them, add them, it's not more complicated than that

jverse I fixed it to defending player. Treasure tokens will definitely have the text, so it should be fine in Plunder. I think the idea of failed voyages works a little bit better as sorceries/instants than chance, since it could just end up destroying your card as soon as it enters the battlefield.