[S1]
1) Found marker: 'Your goal, is to somehow, get inside that tower, however, if you have already done that, than you can just swim to the cliffs'
2) Found marker: 'Check out the stained glass windows,walk to the waterfall, and look towards this tree, when you pick up the next marker'
3) Found marker: ':)'
4) Found marker: 'Swim towards the seemingly distant cliffs, until you find the next marker'
5) Found marker: 'Go into the openning'
6) Found marker: 'Almost there, keep moving forward'
7) Found marker: 'Keep moving forward, and swim behind the next landscape'
[S1 Maintainers Room]
Found marker: 'The next markers give specific instructions for accessing the 4th area, so start taking notes!'
9) Found marker: 'The switch for this door, is the last step to access the 4th area, so keep going to the shell symbol'
10) Found marker: 'Hit the shell symbol during the sound of the rumbling gears, then run towards to the door.'
11) Found marker: 'After the squibs are gone, have someone else continue to next area, while you stand by the shell symbol.'
12) Found marker: 'After the rumbling gears stop, hit the switch(you should hear more rumbling gears), then wait till the sound stops'
13) Found marker: 'Then, link back home, and come back through the shell bookmark'
[S4 Maintainers Room]
14) Found marker: 'Congratulations, keep moving foward'
15) Found marker: 'look down for the ladders'
[Kadish’s Office]
16) Found marker: 'The next marker is a spoiler for the code, if you would rather figure it out , the Ahnonay cathedral has hints on the walls'
17) Found marker: 'Number the symbols 1 to 8, left to right, from the top row to the bottom row, then enter 3,2,1,4,8,5,6,and 7'
18) Found marker: 'After you hit this shell symbol, and finish the age, link back into the first page of the book, not the bookmark itself'
[S4]
19) Found marker: ':)'
20) Found marker: ':)'
21) Found marker: ':)'
22) Found marker: ':)'
23) Found marker: ':)'
24) Found marker: 'Last one!'

There's one inside the tower, in the first area with the squibs. The reason why one of the markers says that you can skip it, is because that one marker in the tower, gives a simple tip for changing the water flow, so that you can swim farther, past the tower. I didn't want the Marker game to be too redundant for those that already know that one tip. However, I still kept that tip in there, for those that are clueless.

'Swim towards the seemingly distant cliffs, until you find the next marker'

I think there's another one in the water, which names the false landscape, as a matte painting

it's touching the front of a false landscape, that is directly across from the tower