Below is a guide for the Shower Investigation. 

If you're having a hard time solving the puzzles and don't think you can work them out by yourself, use this guide as a last-ditch effort to make it through!

SPOILERS BELOW! If you'd still like to try to complete the puzzles yourself, don't scroll down!

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And so it begins.

The shower is simple to navigate, as it is a single room. Pressing left or right will swivel you around from the center of the room.

You can chat with Ethan or Jude if you like. When you're ready to move on, press the right arrow twice to reach the shower screen.

Next, click on the shower and watch the cutscene.

Once the cutscene completes, click the left arrow three times to reach the leftmost angle of the room.

The first puzzle we'll be solving can be seen on the left locker, this color wheel.

Clicking on a color removes it. We'll need to find the order each button must be pushed.
Press the exit button to return to the leftmost screen.

On this screen, click on Ethan's tank top to add it to your inventory.

Next, press the right arrow to advance to the center-left room.

Click on the poster and watch the cutscene.

Next, click on the slime puddle with Ethan's Tank Top equipped and watch the cutscene.

We now have all the info we need to solve the color lock! Press the left arrow to return to the leftmost screen and click the left locker to begin.

The colors available to you are green, blue, orange, yellow, and pink. 

We can determine the order by looking for hints on the poster and bench.

There are two green tally marks, 3 blue tally marks, IV (four) is on the poster in yellow, and 5 (five) is on the poster in pink. Since we found every other color, Orange can be assumed to be 1 by process of elimination.
Orange, Green, Blue, yellow Pink. Hit the buttons in that order and watch the cutscene.

With the locker now unlocked, click it to take a look inside.

Click the beaker and watch the cutscene. It's in your inventory now.

Press the down arrow to leave the locker, and then press the right arrow two times to reach the center-right portion of the room.

Click on the sink. A brief cutscene will play, followed by a short tutorial pop up. Read it and click to continue.

With the large beaker selected, click the faucet. This will fill the beaker up to the first line.

Repeat this process three more times, until your beaker is filled to the fourth and final line.

With the beaker selected, click the right table indent. This will remove the filled beaker from your inventory, and open another locker.

Press the down arrow to return to the center-right screen, then press the left arrow twice to go to the leftmost screen. Press the right locker with the 7 painted on it.

Inside the locker screen, click the yellow screwdriver to add it to your inventory.

With that in hand, press the down arrow to return to the leftmost screen, then press the right arrow twice to reach the center-right screen.

Now, with the screwdriver equipped, click the panel on the right side (partially obscured by the party menu). Watch the cutscene. Afterwards, the small beaker will be added to your inventory.

We can finally solve the last puzzle now!
Click the sink once again.

Click the large beaker to put it back into your inventory. You'll need it for the next puzzle.

Next, with your large beaker selected, click the sink to dump out the water.

Now we'll need to determine what the symbols mean. The first step is easy!
Remember how placing the large beaker filled all the way to the top line opened the locker with the seven on it? With that info, we can determine that the large beaker holds 7 units of water total.

Now that we have a number to work with, determining the other symbols is easy! The beaker has four evenly spaced symbols, the top one being 7. That means we can divide 7 by 4. (7/4=x) This gives us 1.75, meaning that one line is equal to 1.75 units of water.

Let's take a look at the smaller beaker now. Notice anything familiar?
That's right, the small beaker has a symbol in common with the large beaker! The small beaker's topmost symbol and the large beaker's 2nd symbol are identical!

With this information, we can determine what all of the symbols on the small beaker mean, too! To solve for the symbol's value on the large beaker, we just need to multiply one line's worth of liquid with 2. (1.75 x 2 = 3.5)

That means that the maximum fill of the smaller beaker is 3.5 as well!

The small beaker's maximum volume is 3.5 and there are 7 evenly spaced lines. Using the same process as before, we can determine that each line of the smaller beaker is equal to 0.5. (3.5/7)

Here is the final breakdown of beaker measurements for quick reference, using the math we just did.

But how can we use all this information? The answer has actually been staring at us since the very start of the investigation!

We need to, as Rex Lennox says, strive for 5!

There's only one way to reach a total value of 5 with the beakers we have. We need to fill the large beaker up to 3.5 and the small beaker up to 1.5. (3.5 + 1.5 = 5)

That means filling the large beaker up to the second line...

... And the small beaker up to the third line!

Once you've done that, with the large beaker filled up to the correct amount (and selected), click the right slot.

Next, with the small beaker filled up to the correct amount (and selected), click the left slot.

A cutscene will play. 

Once you've finished the cutscene, click the door to complete the investigation... But at what cost?