The storylet at depth 5 gets you a vanity quality and it's strongly suspected that this will coffer a unique item/benefit at some point but I suppose not guaranteed; this Wiki says:

• In addition at least one unique item can only be obtained by diving to max depth. An encounter there will get you Discovered: the Pentamerous Bride, a vanity quality, and you can do this up to 5 times, each time getting 1 x Audiences with the Bride.

HOW DOING STORYLETS LATER IMPROVES FPA

Let’s say you are planning two trips to the bottom, one for favours, one for storylet.

Strategy 1 = 14.3 FPA = Do the storylet first, and in the second trip your FPA is 14.3 as per the spreadsheet.

Strategy 2 = 15.7 FPA =

When you reach depth 5 you have a 58% chance of being offered 400 favours for 16.7 FPA (400/24).

And so a 42% chance of being offered 300 or less; in which case you take the storylet. In your second dive, having done the storylet, you have expected FPA of 14.3 again as per the spreadsheet.

So your “overall” FPA from the second strategy is 58%*16.7+42%*14.3 = 15.7FPA…

That’s with 2 trips, targeting 1 storylet.

But what if you plan 40 trips to the bottom targeting 5 storylets? Well the “overall” FPA changes so much depending on your situation that each case needs to be calculated separatel, I doubt there is a neat solution; but generally speaking the higher the storylets are as a proportion of your dives to the bottom, the closer your FPA (with the 2nd strategy) will be to 16.7. I suspect I’ll do at least 10 favour dives, 5 storylet dives and I’d intuitively guesstimate my FPA would improve by 0.5-0.7 doing storylets only when I have to; so not doing it this way I suspect is not a big deal, but there you go.

Incidentally here is what you can expect at Depth 5

Devotion 11 depth 5 FPA = 14.3 FPA (343 expected favours/24 actions)

This is broken down into

58.3% chance of 400

29.8% chance of 300

7.1% chance of 200

3.6% chance of 175

1.2% chance of coral

There is an 11.9% chance you’ll get 200 favours or fewer, a 1.2% chance of “all coral”; so even though there’s 0% chance of drowning, there’s a small chance you’ll wish you would have.