For your area or all of the PNW? I will mention all of the waters I have been boating on.
For your area (I lived there for 2 years): Lake Washington, Arrive to the ramps EARLY on days with temps above 75 the ramps pack full of people that shouldn't have boats and can't back down a trailer. So if you go after 8am, go with a lot of patience.
Puget Sound: Ramps aren't near as bad, however make sure to check weather forecast for afternoon winds. The wind rips through parts of it and kick up a pretty good swell that can get people in a lot of trouble. Also having a VHF in the sound is very good to have to communicate with the large volume of commercial vessels, or calling for help.
Upper Columbia River (Tri-Cities, WA area): Check out a chart of the river area you plan to boat so you have an idea of where the shallow areas are. The upper reaches by Hanford get very shallow, where Jetdrive is the only recommended way to get there. The beginnings of Lake Wallula above McNary Dam have a lot of shallow areas so follow channel markers, even with jetdrive you can find bottom.
Mid Columbia River (McNary Dam - Bonneville Dam): Again check charts for shallow areas. There are a lot of rock formations that are just below the surface of the water around the edges of the river. Watch for breaking water away from shore to help spot trouble areas. This area is frequently VERY VERY windy. So keep an eye out for the hundreds of wind surfers / kite surfers. Also this area has a decent amount of barge traffic so keep a look out for them, as they have the right of way and will not leave the channel to avoid a collision. So if you are floating in the channel be aware of your surroundings at all times.
Lower Columbia River (Bonneville Dam - Portland): Closer to the dam has some shallows and submerged danger areas. Check charts like usual. Towards Portland/Vancouver the waters are generally good and obstruction free. Keep an eye out for logs. Especially in early spring the winter runoff brings a lot of logs into the river.
Snake River (Between Hells Canyon Dam and Farewell Bend): Water levels here fluctuate A LOT. Especially middle to end of summer. Check water levels and what ramps are open before making a trip out as Brownlee Res. is very low and usually ony has one ramp left in the water by the end of summer. With the variable water levels, rock formations that are clear at full pool are easy to hit when water level starts dropping. Stay away from the canyon walls to avoid them, and go at idle speed keeping a sharp watch for submerged rocks when approaching shorelines.
Snake River (Below Hells Canyon Dam to Lewiston, ID/Clarkston, WA): Rapids. Serious aluminum jetboats with big power, rafts, kayaks, and driftboats are all I would dare to take out there (Which is a ton of fun but feeding fuel to those jets is not fun).
Snake River (Lewiston, ID / Clarkston, WA to Pasco, WA): All dammed up waters. Labeled channel the whole way for barge traffic. Charts are available, but water levels kept fairly constant for barge traffic.
Central Oregon High Mountain Lakes (Cultus Lake): Cultus Lake is located about 45 minutes away from Bend, OR and is the only one without speed limits or noise limits. Very rocky shorelines with the exception of the end of the lake, big cove, and little cove all have nice sandy shorelines as well as camping areas for boaters. Very shallow rocky area by the drive in access camping area (our shallow jetboats clear most of them with one area of exception). Crystal clear water and actually fairly warm by end of summer, but very cold in early months as this lake does freeze solid.
Most other central Oregon High Lakes have speed limits of 5-10 MPH AND noise restrictions, these are all great fishing lakes, but best suited to little aluminum boats with 5-10 HP motors or kayaks/canoes, and rafts.
Some other lakes around Eastern Oregon (I grew up there) Phillips Res. 20 minutes outside of Baker City, OR is a nice lake with decent fishing. Water levels fluctuate greatly each year depending on snow pack, but not much of a destination location if you are looking for boating with a lot of other people.