TBH, 5e hates secret roles. (yes, i know this is not a 5e specific post, just a personal gripe)

There are way too many abilities that trigger on a failed roll or can be used after a roll that this kind of play style straight up conflicts with.

One of the worst ones for this kind of thing is the soulknife rogue’s Psi-bolstered knack (that they get at level 3), with both triggers explicitly on a failed roll and refunds if the extra dice if it doesnt make it succeed so you cant even include mystery if the reroll made it a success.

Dex would be super important for them too. Cannons of all flavors aim with dex due to being ranged weapons.

I have issue with almost all of your points.

• Dex & wisdom would be the important stats for sailors IMO (i am not saying strength and con are useless, but just not as important, they would be 3rd and 4th IMO).
• Darkvision wouldn’t do a damn thing, its range would barely get off the side of the boat.
• Poison resistance is not disease resistance.
• Some spoiled food would be poison, but some would also be disease (depends on how it spoiled and the food).
• In order for short stature to be relevant to ship design, they would need a completely custom designed ship tailor made for them, which would have benifits but sounds unlikely for what pirates would be sailing. (That being said, a dwarven merchant ship designed speciifcally for their stature would be a massive pain for normal height characters to battle on)
• lower speed means they swim slower too which means going overboard is even worse.

EDIT: dwarven submarines!

• short stature actually hugely relevant as submarines are notoriously cramped
• darkvision means no need for lights in the sub, beneficial for multiple (admittedly somewhat minor) reasons.
• honestly no clue what stats would be important for a sub, no argument here
• toxic gasses less of a problem thanks to poison resistance
• Dont even need to go to the surface, a fully underwater submarine port could be connected to existing dwarven settlements near the ocean with the correct design and planning.
• Also benefits from legendary dwarven engineering.

Honestly, if they caught the hint, understood what it meant, and took action, that would be a miracle and would totally be worth some cheese.

You keep doing that to older DnD players too, except you drop hints that the enemies are in a cult that has been giving them power and when the wizard does fireball them, the wizard gets hit by a dozen hellish rebukes from all the young warlocks they just hit.

As written in the spell description of dispel magic, a 3rd level dispel magic also can dispel a 9th level spell with only a check, that didn’t stop them from writing some things that explicitly stated that dispel only works for that effect if cast at 9th level.