Always wondered if you could mitigate this somewhat by basically putting your sat in a bag of water and leaving the antenna and solar panels sticking out.

Not really. Radiation shielding has diminishing returns with thickness as the relationship is logarithmic. A few millimeters of aluminum cuts down most of your ionizing dose by orders of magnitude over unshielded, but doing appreciably better requires impractically thick shields.

And that only helps with ionizing dose, which is already not really a problem in LEO. The issue is more high energy particles like cosmic rays, which cause single event effects (SEEs) - things like random bit flips in RAM or CPU registers, or transistor latchup that can cause destructive shorts to ground if not mitigated. These are impractical to shield against, unless you want to fly a few feet of lead. So instead we mitigate them (ECC memory, watchdog timers, latchup supervisor circuits that can quickly power cycle a system to clear a latchup before it can cause damage, etc).

If you want to get an idea of how much shielding is effective in a particular orbit, you can use ESA's SPENVIS software (online, free): https://www.spenvis.oma.be/. Despite being free, it's the tool of choice for initial radiation studies for many space missions worldwide.