Honestly, I’m not sure how major a gaffe forgetting them was? Maybe a bit embarrassing, but it isn’t like they couldn’t have simply called the Cerritos in a normal situation to send them in a millisecond.
Plus, it is the first time I remember an Away Team not carrying them on each individual.
I think we have to take it as a premise of the episode that for whatever reason, part of Mariner's duties were to obtain tricorders for the team and bring them down with the team, even though we have literally never seen anyone distribute tricorders during the course of the mission in all of Trek.
Let's analyze Mariner's actions from an ethical point of view:
1) She did something which benefited only her while she was on a job? Yes.
2) The actions, which benefited her, distressed other people and/or made their lives or job more difficult? Yes.
3) Did she lie to other people who trusted her to benefit herself? Yes.
4) Did she repent or apologize for her actions? No.
I rest my case.
It is AN ethical point of view, not the only point of view, best point of view or even necessarily a "correct" point of view. I could spend a fair bit of time to quibble with each of the points either as to whether that is a valid criterion,whether it actually applies to the situation, or whether other considerations should be factored in.
For instance, I don't think it's fair to say that her taking a dive only benefitted her. It at least arguably benefited (or was intended to) Ramsey by preventing her from being in a position to have to be rejected. It benefited Ramsey's regulars from having to deal with the notion that Ramsey would prefer Mariner to them.
Is lying, even to people who trust you, inherently unethical? A philosopher like Kant would say yes. Most people in the real world regularly lie or lie by omission numerous times to people who trust them.
Along similar lines, it is unclear that Mariner needs to "repent or apologize" for what she did, and to the extent that she might, it is clear at the end of the episode that Ramsey and she have reconciled over the issue. So it seems to me either a. Mariner did fully apologize off-screen b. What Mariner said constituted enough of an apology for Ramsey for them to move on c. Ramsey doesn't think that Mariner needs to apologize for what she did because Ramsey doesn't see it as a big deal.
But at the end of the day, I don't think for most things ethics can be reduced to a black/white, either/or situation or set of criteria.
Using these or similar criteria, most of the crew of the main characters aboard have acted unethically. The one exception might be T'Ana, who has not been shown to lie.