When the IMU is at rest, it reports 1 g acceleration along a strictly vertical axis.
That is all you need to work out the scale factor for conversion to g values.
Use 1 g = 9.8 m/s/s to convert to m/s/s.
For the most accurate measurements, you need to calibrate the accelerometer to remove offsets and correct the scale factors for each axis. This is the best and most comprehensive tutorial: Tutorial: How to calibrate a compass (and accelerometer) with Arduino | Underwater Arduino Data Loggers