For the cable entering the j-box I use a waterproof cable clamp. I pick them up at my local electrical supply house for about $3.00 each.. Well worth the expense. They work best if you use a round cable. If using 4GA or larger cable each wire gets its own water tight entrance to the j-box.
Most marine wire is not UV stabilized. Despite the lack of UV inhibitors it can still last in the five to ten year range out in the sun, depending upon where you are. A UV stable SOW, STW, STOW, SEOW, or STOOW type cable can work well and can often be ordered in sizes up to 6/2. These cable types are the same or similar jacketing used in shore power cords and they hold up quite well in solar applications..