I would go with the Silver. Having the LCD screen to quickly review video and check camera angles is nice. The GoPro app is ok for this but it is laggy and requires you to connect your phone to the GoPro over WIFI. How quickly the SD card fills up depends on the resolution and frames you decide to film in. You only need high FPS if you plan on doing slow motion. The more frames will make super slow motion smooth as butter.
Here's a link to GoPro that will give you an estimate of how much a 64GB sd card will hold in each setting.https://gopro.com/support/articles/hero4-silver-recording-time-in-each-video-setting
As far as battery goes. I use 2-3 for an 8 hour day shooting in 1080 at 60fps. This will vary depending on how often you have the screen on, etc. If the boat you are going on has a cigarette lighter, then just bring a charger with a long cable and you won't have to worry about it.
Biggest piece of advice I can give you would be to stop recording after any moment that you want to find easily. For instance, if you catch a giant, after you release it and say whatever to the camera, stop recording and then start recording again. That way you know the footage is at the end of that file. It will save you a huge amount of time instead of scrolling through 40 minutes of footage trying to find the 1 minute you want. It will make editing a breeze.