Ok, that's how to get in, but not really how to get a data out :)
Water in and of itself isn't a great communications barrier, but the conductivity of sea/salt water causes it to act like a Faraday cage and kills/significantly dampens any radio waves attempting to pass through it.
Under the assumption that Europa's oceans are in fact salty, the frozen sea ice isn't actually all that salty. Ice in general isn't a good conductor as it's actually almost entirely the pure water that freezes leaving the salt behind (I'm not clear on whether it would precipitate but remain suspended between the the H2O molecules or if - given the lower density of Ice I - the water would freeze from the top down leaving the liquid seawater beneath it ever so slightly saltier while forming a layer of pure frozen ice on the top).
In all cases, keeping a base station while sinking a second module with the radioactive heat source then proxying all communication via the base station would be possible even if the ice did form some form of a Faraday cage that severely limited communication from beneath.
In any case it seems like there'd be astronomical expenses and many moving parts
I've thought about this as well. I think there are "Prime Directive" type concerns with this approach though.
Thinking that soon or later (after 1/10/100/1000) years the probe will for any reason release its radioactive payload into that environment, therefore contaminating it. (kind of similar as well if it keeps generating warmth at least partially for years and years and years => maybe in that case bacteria might start to increase/evolve because of that warmth, and any later observation would have to take that effect into account, which might be difficult to separate from all the local&natural causes - or it could be as well the opposite, with the probe "burning"/killing existing bacteria because they're not used to that warmth)
something something Couple 'o Raptor rockets bolted onto a methane-harvesting sled for fuel, let 'er rip ..
/there I fixed it.
It'll be the steam-cleanest Starship-converted-to-ice-submarine in the system ..