Hey, I gave the game a play.  Here are the answers to your questions:

1. 7 days.  I had gotten all of the upgrades, so it didn't seem like there would be anything new to do

2. Yes.  I'm a sucker for upgrades!  Honestly if you just let me keep upgrading I probably would have played longer just to see how high I could get them.  The balance between upgrades that create more resources vs upgrades that let you harvest the resources faster is good.  I would lean into that.  Reminds me of this itch game I played before: Box Box Surprise by Happy Distractions Studio

Also, by the time I maxed out the upgrades, things were really satisfying, where I could just click and the things would break almost instantly.  It would be fun to move forward with that with a lot more oysters coming, and upgrades costing more and more.  And maybe you need new types of upgrades to battle rival otters or something.

One note on the upgrades, though, I actually missed that there was that option on the first night.  You might want to make that clearer, maybe with a text popup or something

3.  5 stars (see above).

4.  Not really.  I didn't have any preferences for particular upgrades, other than I just knew I wanted them to make the clicking more efficient.

5.  See above the note about missing the upgrades on the first night.  Otherwise, I didn't encounter any issues or bugs.  Nice work!

1. How many days did you play before stopping? What made you stop?
8 days. I had already purchased all the upgrades and it didn't seem like there was more content yet beyond that point. 

2. Did the day/night cycle make you want to keep playing into the next day? Why or why not?
I think with a bit more visual polish and a bit more going on (e.g. something new happening each day), it could be quite enticing down the line.

3. Did the night upgrade phase feel like a rewarding break, or did it interrupt the flow? (rate 1-5 stars)
I found it to be a nice break from clicking. However, I can see how in the long right it might feel a bit frustrating having to complete a full day-night cycle to purchase upgrades. But maybe I only feel that way because most incremental/clicker games like these give you the choice to purchase upgrades right away as long as you have the resources on hand. I can appreciate that this game is going for a different kind of pacing.

4. Did you find yourself looking forward to anything such as a specific upgrade, the next day, or something else?
I guess I was hoping for the numbers to go up at a faster rate with each upgrade. 

5. Any bugs, confusing moments, or frustrations?
At first I was a bit confused on what to do. I had clicked on one of the shells floating by and failed to notice it was on the otter's belly and that I had to click on it to break it. I also failed to notice that the stars that appeared at the end of the day were upgrades I could purchase.

Closing Remarks
It's an adorable spin on the genre, and I can't wait to see how you'll expand on this in future updates! Maybe it's because what we have here is a small vertical slice, but I think you can afford to make the game ramp up a little faster, as well.