Very good idea.
In deep-water plays and in order to calculate the reservoir temperature (if you don't have actual data):
Consider the seabed temperature is 4 dec C (rule of thumb) for any ocean or sea unless you have real data.
start counting from seabed downward to the reservoir .i.e. subtract the water depth from the total depth in -MSS and apply the regional geothermal gradient.
If you have a well targeting reservoir A with depth of 4000 meter SS
And water depth of 1500 meter
and the geothermal gradient is 5 C/100 meter
Then 4000 - 1500 = 2500 meter (depth from seabed to reservoir A)
2500 * 5/100 = 125 C (incremental of temp.)
So, the temperature at on A depth = 125+4 = 129 C.