Seamark imports tracks as GPX files. If your track was recorded by another app or a chartplotter, it has to be exported from there first. Pick what recorded yours; each guide goes from the recording to a .gpx file.
With the file in hand, open the leg in Seamark and choose Upload GPX track. Seamark drops the GPS glitches, works out the distance, times, speeds and night hours, and draws the track on the chart.
User manual