I have had a similar issue where droplets of water would form inside the LCD window. Really not sure how and why that happened. Maybe the camera was not handled properly when passing on to the boat or maybe while traveling( i always carry it as cabin luggage in my haversack). There was no discernible crack but water was getting in, albeit at a very slow rate. i sent the housing (Nauticam for Canon7D) to the dealer and Nauticam Replaced the LCD plexiglass. Post that no issues.
Some of these wet lenses are manufactured that there will be no vacuum between the port and the lens so even if you screw on the lens on top water "will" enter between the two as there are holes provided. They are also made to specifically have a layer of water so a vacuum will not work. Inon has a wet lens with a dome in front that works with a vacuum between the two. So in such a case water will NOT enter. so depending on the lens is the use.
As for your second query, strobes are your best option for stills. Lights are no substitute. Two such lights will work for stills BUT you will need very large apertures and slow shutter speeds and the results are nothing as compared to strobes. They will work well for video though. There are enough discussions on this on WP so check that out.