Agree weight the boat at a scale--cost less than $20 at any truck stop which has certified scales--pretty good insurance,--if more than 3500#, you could tow from a point on a storage lot to a ramp... (Ok I am being over cautious--you can do that with a 15 hp lawn tractor, if you could keep the front wheels on the ground.)
Many of the boats come with undersized trailers single axle for road use. I got away with it for 3 years and over 15,000 miles--until the trailer broke...than I got a 7500# capacity trailer, and no problems.
You need to monitor the transmission temperature--that will be the weak point in your drive train. Get a SCAN G gauge, which plugs into the OBD port on the vehicle, if your SUV does not already have a digital transmission monitor. You live and boat in Seattle, and no hills? You say no mountains--which I assume means that you stay out of the coastal ranges, but as I recollect there are some significant hills in the Seattle area...try and avoid those also.
The biggest issue is not go--it is STOP...Be sure that the trailer has very good surge brakes. The idea is to be safe, and not damage the vehicle, or significantly shorten its life span. There are ways that vehicles and hitches can be upgraded to carry more weight--but you might run into an insurance if there is an accident.
The Lexus RX-330 is a unibody vehicle, so there is no frame to beef up, and thus no way of increasing the hitch capacity. I also towed a C Dory 22 some relatively short distances with a Honda Pilot. Sure a lot safer and better driving experience with a truck frame and weight. I happened to weigh my GMC Yukon XL yesterday: 6150#. Definitely more weight than the Pilot or Lexus. (both slightly over 4000#)