The cache is not at the posted coordinates, Brother Michael did not intend for you to go swimming in Strangford Lough. In order to get the correct coordinates you should complete the Adventure Lab - The Lost Treasure of Inch which should take about 2 hours to complete. You will need to download the Adventure Lab app to complete the adventure.
On completing each stage at Inch Abbey, Saul Church, Struell Wells, Nendrum and finally Movilla Abbey, you will receive a number which will enable you to work out the final coordinates
The cache can be found at
N 54 35.A B D
W 005 40.C D E
You can use the solution checker to see if you have the correct coordinates.