It's not exactly 7 tiles with 100 swimming, it depends on your encumbrance and on your endurance. It will vary from character to character how far you can safely go at 100 swimming.
Yes you can probably make it, if you endurance is good and you're unencumered -- you want to be naked for the attempt, you can come back for your stuff later once you have a raft.. To be sure, Try swimming diagonally from the bottom right of the tile with your shelter, to the tile northeast of it, and then back again. Count how many laps you can do diagonally.
The fatigue increase is not very random - you always get 1% for the first step, but after that it shoud be fairly consistent to 2%, then to 3%, and you will always get the message "you feel moderately fatigued" right at the halfway point -- if you go in a straight line and get to "moderately fatigued", you can turn back and reach the shore safely. 5 tiles is 320 steps, so see if you can do 160 steps before you get to "moderately fatigued" before making the attempt.