Can I put charts dowloaded from Navionics on my F6 Pro?

No way. Here is a link to the Garmin Charts. Or search for openseamap.

maps.garmin.com/.../

If you don't mind doing a little extra work, you can download free marine charts from CalTopo to use on your F6.  Their free accounts limit the size of each downloaded chart area, but you can download as many of them them as you need to cover your area of interest.  They have a lot of basemap and layer options to choose from too. 

I've used their site to get really detailed topos for hiking (10ft contours!) and marine charts for multiday sea kayak trips.  If you need higher quality maps than Garmin offers, and for only a little time and effort, it's well worth making your own custom maps.

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For those who are looking for more detailed instructions on the entire process:

1) Go to the CalTopo website and configure the map the way you like (select which map types, layers, etc. you want displayed)

2) Select the 'Print' button along the top of the screen and then select 'Download KMZ / MB Tiles'

3) Select the area to save by dragging the red arrows or move the area around using the red dot in the center (For the free version only 16 tiles can be selected per download)

4) Click the 'Download KMZ' link on the left side of the screen.  Give the map file a descriptive name so that you will be able to recognize it on your watch (e.g. 'Marine 5 Blue Hill Bay.kmz').

5) Repeat steps 3 and 4 until you have downloaded enough maps to cover the area that you need for your trip or activity

6) Connect your watch to your PC via USB and drag and drop the map files into the \GARMIN\Custom Maps directory

7) Disconnect your watch from the USB.  It will restart and load your new maps.  The maps will be enabled by default, but you can disable / enable them individually by opening the Map activity, long press the Up/Menu button, then go to Map Settings > Map > Configure Maps > Custom Maps

Hopefully this is helpful to someone. :-)