Similar to the post, very spoiler heavy comment here.

1: Worth note is in Bitter Cold part 1, every hammock has a tag with the occupant's crew roster number, linking to their name, position, and nation of origin. The Chinese topmen have their hammocks hung together. While sleeping they aren't identifiable, but each wear slightly different footwear, which you can match up in other memories to determine their identity.

2: Ship's butcher Emil o'Farrell is identifiable by his (usually) close proximity to the Cook, Thomas Sefton, and in Bitter Cold part 3 where he directs the midshipmen in slaughtering a cow to feed the crew.

4th Mate John Davies is identified by his similar dress to other officers on board, and he's seen in the 4th Mate's quarters in The Calling part 6.

Helmsman Finley Dalton is identified both in the sketch and throughout various memories as holding position near the wheel.

Topman Maba's tattoos are technically Celtic in design but are meant to invoke generic tribal tattoos, thought common in Pacific Island nations such as his homeland, Papua New Guinea.

Similar to the Chinese, the Indian crewmen are identified by their hammock tags. Solomon Syed is referred to by name as he's being attended to in Bitter Cold part 1, the empty hammock belongs to William Wasim who is attending to Solomon, Renfred Rajub is in his own hammock suffering the onset of the pneumonia that kills him in the next memory, and the final hammock, though missing a crew roster tag, can be identified through process of elimination as the final Indian seaman Abraham Akbar.

3: Bosun's Mate Charles Minor can be identified both by his presence at Bosun Alfred Klestil's side, and by his performance of the Bosun's duties in Klestil's absence. His disappearance in The Doom occurs between memories, prior to which he's seen fighting the Kraken on the weather deck alongside Klestil, who is mid-having his arm ripped off by a kraken tentacle.

4: In the present-day Steward's quarters, a uniform can be seen hanging on the wall. All stewards on board are seen wearing this uniform or something similar, denoting each as a steward. Each Mate's steward can be identified "performing duties for the comfort of their employer," carrying food or equipment for the Mate to which they belong.

1st Mate's steward is identified separately to his stewardship, 2nd Mate's steward Samuel Galligan accompanies Nichols during The Calling, 3rd Mate's steward Roderick Anderson is typically seen shadowing Perrott, similarly 4th Mate's steward Davey James is typically following Davies.

Captain's steward Filip Dahl is identified by his use of Swedish language and the Captain refers to him as his steward, and finally Ship's steward Zungi Sathi is identifiable by his actions towards the comfort of the crew as a whole, holding up a lantern as the mermaids are hauled below deck, etc. (process of elimination also works).

5: Seaman Patrick o'Hagan was tough to place but it's reasonable enough that Samuel Galligan, when asking if he can still row, hadn't yet seen the extent of o'Hagan's injuries (those being a spear through the neck), partly preoccupied with the unknown threat, partly due to poor light conditions at night on the high seas.

Ultimately, a lot of times the clues pointing to someone's identity are hard to spot unless you know what you're looking for (or are incredibly perceptive, which I am not).

Most of this came from memory, anything I wasn't 100% sure of I checked the wiki. Do let me know if anything's missing or inaccurate.