Roger,
...my plans are to attempt some more serious landscape/seascape work...
For seascapes you may find a reverse graduated filter handy, although watch out for flare when shooting into the sun with the filter in front of the lens (of course this applies to any filter). Singh-Ray sells them.
Your polariser will come into its own for seascapes because colours already close to complementary will be saturated, it will make clouds pop, cut glare from the surface of water allowing marine detail and colour to be seen beneath. However, take care to not overdo the effect, and watch out for banding of the sky which will be increasingly visible with focal lengths wider than approximately 24mm (35mm equivalent). Note that this last point is irrelevant when shooting in a forest, for example).
Singh-Ray sells a Gold and Blue polariser, which some love and others hate.
LR has allowed me to “skate by” without learning much about ND filters or alternatives.
Nothing wrong with that. In fact, unless you are particularly keen on making as much of the photo at the time of capture, thereby reducing the amount of post processing, there is a strong argument for applying filters digitally: it will save you financial expense; you will have no issue with subject matter passing through the transition of a graduated filter; you will reduce the risk of flare when shooting into the sun with a filter in front of the lens; and a mistake is easily remedied without risk of ruining the photo.
If you do decide to purchase graduated filters I recommend a 2-stop soft or 2-stop hard, and between the two, the former before the latter. Why 2-stop? Because except in extreme circumstances, the dynamic range of your S is unlikely to be 3-stops short of the scene requirements, and because a 1-stop filter hardly seems worthwhile - the image will not be ruined if you expose for the highlights and have the shadows blocked up a touch.
For seascapes, and likely to a lesser extent landscapes, a neutral density (i.e.
not graduated) filter will help increase exposure time, which of course will 'smooth' wave action or return the classic silky smooth waterfall effect. With landscape, clouds will be softly smeared. When looking to achieve any of these effects, it may be worthwhile purchasing a
variable neutral density filter instead of of one or more x-stop filters, because the shutter speed required to achieve the effect you are after will vary as a function of the light intensity...and a variable ND can accomodate that conveniently in a single filter.
Hope this helps.