Underwater excavations calculations are dependent on:

How often can you dig
How many bones you are finding vs. Relics and other items

For example - on Preprod, even after using the equivalent of 40 passes total, and only missing a handful of digs (I had enough Relics to trade for the next tool as soon as able) - I was STILL short 30 - 40 something bones by the end of the promotion.

For those who want some numbers...Here's another rough example for a different run of the event based on 1-day sample results with each tool based on MY experience *you may find more or less bones, relics, traps, prizes, etc*

Snorkel and Mask (10 digs)
8 Relics
2 Bones
0 Prizes
0 Traps

Oxygen Tank (15 digs)
10 Relics
4 Bones
0 Prizes
1 Trap

Diving Suit (20 digs)
10 Relics
6 Bones
2 Prizes
2 Traps

Propeller (25 digs)
13 Relics
8 Bones
2 Prizes
2 Traps

Robot (30 digs)
19 Relics
10 Bones
1 Prizes
0 Traps

Now, let's focus on the Bones gotten. We'll assume with our other digs, we got enough Relics to barter for the better tools for the various amounts of digs per day for these dates *dates are preprod dates NOT live dates*:

Days 1 and 2 (13th and 14th): 10 digs per day *Snorkel and Mask*
Days 3, 4, and 5 (15th, 16th and 17th): 15 digs per day *Oxygen Tank*
Days 6, 7, 8, and 9 (18th, 19th, 20th, and 21st): 20 digs per day *Diving Suit*
Days 10, 11, 12, and 13 (22nd, 23rd, 24th, and 25th): 25 digs *Propellor*
Days 14, 15, and 16* (26th, 27th, and 28th*): 30 digs *Robot*

*Day 16 (28th) is not a full day so these numbers might be slightly off

So from the above results:
Day 1 - 2: 4 Bones
Days 3 - 5: 12 Bones
Days 6 - 9: 24 Bones
Days 10 - 13: 32 Bones
Days 14 - 16: 30 Bones
Final total: 102 Bones

Bones needed for Devoted Indigo: 162

That's a difference of 60 bones that must now be made up with Mines.

Looking at my history with Mine usage in a single day (3 mines for 1 pass):

2 Bones
1 Prize
0 Relics
0 Traps

If I continued to have this luck with single mines...I'm looking at 30 passes to make up my missing 60 Bones (2 bones each time ; 2 x 30 = 60).

If we calculate this with my results based off a run with Double Mines (6 mines for 1 pass):

3 Bones
2 Prizes
1 Relic
0 Traps

This comes out to only needing 20 passes (3 bones each time ; 3 x 20 = 60)

Final potential pass cost based on these numbers: 20 to 30 passes

Again though, this is just based off a very small sample of numbers and the assumption that one will get these amounts every dig and have enough relics to barter for the next tool as soon as able and overcome traps given to them in the same day.

In short - for the Underwater Excavations - expect it to require anywhere between 10 to 50+ passes depending on your luck with digging.

does any one know if we will find extra digs or something a Horn of plenty or a Titian's challenge?

what are traps

ShorahNagi, on your wonderful explanation you did not mention digging for a 2nd level. In prior excavation promos we often had to dig to a 2nd level, or is that only possible with a mine, I don't remember? The last exacavation was quite awhile ago. Also, when do the 2nd levels start, right away or farther into the promo?

The 2nd levels start right away, I just dug down to one and got another relic. I don't remember though if there are more than 2 levels or not. As I mentioned above it has been awhile.