Also, new NOAA raster and vector charts were recently released so you may want to update them at the same time. If you haven't been keeping up then there may be other files that need updating (particularly the change from US 3D to North American 3D, and the update to version 2.11 of the system).
Attached is a list of the current (free) charts for NN3D MFDs. It should help you make sense of the files your MFD shows installed, and you can quickly determine if you are current. I fill out the 3rd column with an "X" to identify those that I have installed. I have made no attempt to list the non-free charts so you may have other charts installed/unlocked on your MFD.
I find tidal current files for several EU countries on the MapMedia.com website, and they show compatibility with both MaxSea and Furuno NavNet3D. AFAIK both MaxSea and NN3D use the same MapMedia format, and when both NN3D and MaxSea exist on the same system they can share access. As I don't have MaxSea I haven't dealt with the details of sharing. For the MFD8 you can download the tidal current files, as with any chart file, load it on an SD card and install it on the MFD8. You will also need to purchase the DRM file (requires your system ID) and install that as well. If you plan on using it with MaxSea as well then you will need to research that (or wait until someone more knowledgeable can respond).
When installing two MFDs in the same network, it appears that both MFDs get the same System ID and thus should be able to install and unlock the same chart and DRM files (if I were you I would confirm this before acting on it).
Sorry for the confusion - I meant the unlock keys. I'm not remembering whether they are in a file or just a text string. "DRM" means Digital Rights Management, a generic term for the technology for controlling access to copyrighted works. In this case the unlock keys are created specifically for your System ID and a particular edition of a chart package when you buy the key. Although MaxSea uses the same MapMedia 3D chart file format as Furuno's NavNet3D and TZ MFDs it needs its own key matched to your copy of MaxSea. (The chart file is encrypted, and these keys prevent unauthorized distribution and use - the chart files can be downloaded but can only be used with the appropriate key for the Furuno system or MaxSea installation.)