Disney will transfer luggage between Disney resorts but not ‘good neighbor’ resorts like the Doubletree. So, you’ll need to bring your luggage to the Starcruiser yourself. As PP noted, they will transfer your luggage to Riv. Note, generally when Disney transfers luggage the rule of thumb is that it won’t be available until 4pm or later at its destination. You see reports of earlier and some later… but the common advice is make sure not to transfer something you will absolutely need (eg, medications, toiletries, swimming clothes, etc) during the day since you can’t control the arrival time and it is likely to be later in the day.
While it’s walkable, there are no guest walking paths from the Starcruiser to the DHS entrance so I’d be surprised if they’ll let guests just walk out of the Starcruiser grounds. Guess we’ll have to see but I suspect they won't allow that.
Getting to a park using Disney transportation is going to be tricky. Since there are no buses to the parks from Disney Springs, you'd need to take the bus to Disney Springs then take a bus to a resort, then transportation to your desired park.
I haven't thought too much about our arrival day plans yet, but on departure day we're heading to our new resort (we're driving) and will make it a pool day. A water park day also sounds like a great idea in my opinion.
Hope that helps!