I was on the Queen Mary 2 in the Mediterranean for 12 days and connection was ok. I even used my Tradestation, which needs a lot of power, watching about 200 stocks realtime. A bit slow but ok.
Swingtrading with 60 min. data I logged in 2 or 3 times during market hours for about 20 to 30 minutes each time primarily watching a limited number of about 20 stocks, mainly smoothly moving ones plus EFTs like DIA, SPY. Didn't trade the wild ones while at sea.
Beeing online is pretty costly though. 50 cts. per minute, whereby one can buy a number of hours in advance to reduce the minute price.
Good sailing
Felix