Well you don't have to really heat it, it's just room temperature. The kind of density you are talking about is only reached well below 100K.

So here is my take on your estimate:

First of all, I was thinking more about an RV-sized gondola. So something like this [0].

The axles are rated for 30000 lbs together, so let's say 15000 kg fully loaded. This includes ridiculous stuff like marble countertops, so there is likely a lot of headroom for trimming weight.

Taking that into account, and the room temperature helium density, we get 15000 m^3 of volume.

Then I would argue that taking a sphere is really not the right approach, because it has poor drag coefficient and also having this elongated shape is what gives you control authority (try steering a circular boat with a rudder, any rudder you apply will just make you spin). The argument about crosswinds also does not really apply because you would always point the nose at the apparent wind, not dead at the direction you want to go. Same as a boat in current or an aeroplane. The exception are gusts of course, and I do see those could pose a problem.

So let's take an ellipsoid with a 1:2 aspect ratio. you need semi major axes of a=b=12m and c=24m to get ~15000m^3 volume. So this thing will definitely not fit in your garage, but it will fit into a pretty standard barn that any rural construction crew builds routinely.

The frontal area is then pi*(12m)^2 = 452 m^2. The drag coefficient for such an ellispoid is somewhere around 0.1 from what I can tell. Can get even lower for a proper airfoil shape but let's go with 0.1 to account for stuff like the gondola etc.

For 20m/s, (way faster than any water-based yacht can travel) gives a force of

1.225 kg/m3 * 0.1 * (20m/s)^2 * 452 m^2 = 22 kN

and a corresponding power needed of

sqrt((22kN)^3 / (1.225 kg*m^-3* 10 * pi * 1m^2)) = 0.5 MW = 700hp

That seems like a very realistic power requirement for such a vehicle, the RV I liked to earlier has 600hp. Might have to be a bit more because the prop area is perhaps a bit much here, but that scales linearly so I don't think thats a huge problem.

Of course if you want to maintain TAS much above 20m/s things escalate quickly. For 40m/s:

1.225 kg/m3 * 0.1 * (40m/s)^2 * 452 m^2 = 88 kN

sqrt((88kN)^3 / (1.225 kg*m^-3* 10 * pi * 1m^2)) = 4.2 MW = 5200 hp

which does indeed seem excessive. But I think for such a craft 20m/s would actually be acceptable. Can't fly it in a storm and can't compete for speed, but both those things are true for leisure sailing yachts.

[0] https://www.entegracoach.com/motorhomes/cornerstone/