Is this your only means for electronic navigation? If so, this would be a strongly argument for installing a marine MFD.
An application crash really only occurs for a few reasons. If the OS and application are both up to date, I would do the following:
1) Backup all my data on the iPad.
2) Do a factory reset to get the ipad back to when you opened the box.
3) Install JUST the navionics software.
4) Install your maps, routes, etc.
5) See if the issue recurs.
IF the issue is still occurring, I would look for another application. I'd also be buying an MFD if I didn't already have one. Incidentally, I use a Raymarine display and an iPad. The iPad is horrible in bright light and with polarized lenses.