Measuring the balance of a rowing boat
Yes, an accelerometer (or two, or three) is what you need to implement an inclinometer.  In a quick search I found a three axis accelerometer breakout board suitable for interfacing with the Arduino (since you used the &quot;Arduino&quot; tag on your question).  To report the inclination angle, you would have to apply the appropriate trigonometric functions to calculate inclination from the reported acceleration values.  Some three-axis accelerometers have built in inclinometer functionality in that they are capable of doing the trig calculations internally.
I’m not sure what effect that the acceleration from rowing might have on the results… perhaps a little analysis would be needed there.
Or, from the purely mechanical side, would a bubble level of some type or perhaps a pendulum like hanging weight be appropriate?
Disclosure: My employer manufactures the accelerometer on the breakout board linked above.