If you are doing vertical conversions over the ocean you really need to use VDatum.
Vdatum runs a detailed, unstructured mesh tidal model (ADCIRC model, usually), then computes tidal coefficients, generates an 18.6 year time series, then determines surfaces of MLW, MLLW, MHL, MHHW, etc. These surfaces are then evaluated at known tidal gauge locations, and the deviations are used to fit another surface which is then used to adjust the grids so there is no error at the gauge locations.
In the final step they intepolate to a regular grid, which forms the basis so the software conversion. But you can ask for the original surfaces on the original mesh (usually triangular) as well. I did this when I was correcting datums in the Gulf of Maine.