Hi Wis,

The boat runs like a submerged drive on 7 cells should, speedwise.
Your motor may have a sticker on it saying 700, but judging by the size of the cells, it's a 600 at best.

You need the caps to keep interference low.

The second pic of the boat's interior holds the answer to your troubles; it's a mess, cable wise!

Keep all powerlines short and away from the antenna, receiver and servo/esc leads, as far apart as possible, I suspected as much from the video; when the boat gets further away from your transmitter, the interference from powerlines takes over and messes up a clean receiver signal.

Tidy up your cables and the boat should run fine.

What size prop do you run?
It looks a tad big.

Regards, Jan.