If there is tilt on two axes simultaneously, then one of these two equations is not correct.
roll = (atan2(-accY, accZ)*180.0)/M_PI;
pitch = (atan2(accX, accZ)*180.0)/M_PI;
The result depends on the order that the rotations are applied, and there are unfortunately six different conventions, additionally combined with ambiguity of the sign of the rotation.
I prefer the pitch and roll definitions in this tutorial, which uses the "Rxyz" convention described therein.
roll = atan2(y_Buff , z_Buff) * 57.3;
pitch = atan2((- x_Buff) , sqrt(y_Buff * y_Buff + z_Buff * z_Buff)) * 57.3;
The two tilt angles can be converted into a single tilt, with an axis that has some determined direction with respect to X,Y,Z axes of the accelerometer. However, the conversion again depends on exactly how you define pitch and roll, and the order in which you perform the operations.
When you have sorted out which definition you will be using, come back. Some trigonometry and algebra will be involved.
Edit: There is another approach to calculate a single tilt angle, described in this much better and more complete reference to tilt calculations. See section 4 "Calculating the Angle Between Two Accelerometer Readings".
To use this method, you will first have to obtain the three accelerometer axial measurements when the device is perfectly upright, which could be done on land. Then it is a simple matter to use the three axial measurements obtained when the device is resting on the sea bottom, to calculate the tilt angle of interest.