You're correct in thinking that a WR kit may be a good idea on a boat, and that the DA*50-135 will give you nice results on any current WR Pentax body. And a portrait situation is exactly where the DA*50-135 can shine. But you may still need to go a bit wider. If you get a reasonable price on a kit with the DA18-135 you can try it out to make sure. The 18-135 still takes good quality shots, just not as striking at the 50-135 does. You could later add the 50-135 and sell the 18-135 here for about what you paid for it as part of the bundle But either way you'll eventually want to have something to cover that range from ~18-50mm as well.
I just put my DA*50-135 on my K-5 to try this quickly. At 10 feet away at 50mm in portrait orientation you'll get most or all of the body of one or maybe two people. In Landscape you'll get the upper half of 2 or 3. Obviously if you can move back a little you'll be able to get more in the frame.
It's hard for me to understand exactly what your needs are at this point. But these are the lenses I think you may want to be considering:
DA18-55 WR (but only average Image Quality)
DA18-270 (average to good IQ)
DA18-135 (good IQ)
DA*60-250 (very good IQ)
DA*16-50 (very good to excellent IQ)
DA*50-135 (excellent IQ)
DA*300 (excellent IQ)
I stick with the 'excellent' or 'vg to ex' IQ lenses. Many people really like the DA*60-250, and it may be a great solution for you, but its images don't captivate me like the DA*50-135 and DA*300 do. Hopefully you've had a chance to do some of your own evaluation - just find the images that capture your attention:
PENTAX : Select a PENTAX interchangeable lens camera or a lens model
As long as you can afford to change lenses (in terms of taking the time and dealing with the weather) I think your ideal kit would be the DA*16-50, DA*50-135, and DA*300. You can even get the K-30 to save a little money while sacrificing almost nothing in IQ. If you don't want to change lenses then either deal with the more limited range of the DA*50-135 (which is often worth it because of the IQ) or get the 18-135 or 18-270 (which, being a Pentax version of the Tamron model, is probably still the best ~18-200+ lens available today).