Petroleum jelly (Vaseline) works, but the clean-up is harder than with emulsifying ointment which will more easily rinse off with water.
Emulsifying ointment is the base for most ointments and can be bought from most chemists/pharmacies in half-litre tubs for just a few quid/dollars, but you'll have to ask for it.
Emulsifying ointment is also easy to customize (make your own ointment) in the home, using just a pan of hot water. A disinfectant (dettol, for example) is a good idea to dope the ointment with as a half-litre tub will last a long time and an antifungal/antibacterial will make it last longer, be softer and help protect the skin from infection.
In such a form, it's also ideal as a shaving cream, for application to spots and cuts and grazes, and so on.
I have diaper rash cream that is 5 in one, it soothes, acts as a barrier, repairs skin, softens and I can't remember what the last one was, lol!:laugh:I only use it for rashes though I don't bother using it as a barrier.
Wanting something that keeps away water and that is easily removed with water are competing goals. You won't find anything that does both well.
Vaseline has other side-effects too. It's been my experience that it outright dissolves the adhesives used inside diapers. Elastic strands come out of leak guards (that can hurt btw), leak guards separate from the inner liner, the inner liner can separate from the shell at the edges, and strands of elastic can come loose in the leg gathers. Vaseline is a wrecking crew for a diaper if it gets to the edges where everything is glued together.
I got a really sudden annoying rash on the inside of my right thigh last week, from working in a hot and humid area for too long. My skin went from fine to on fire in the space of about an hour, it was insane. I ended up with a patch about 1" x 3" that looked sunburned. I tried various treatments to deal with it, but I was somewhat limited by it being outside my diaper's leg gathers by about an inch - anything water-based would get on my pants. So I went with vaseline. Fortunately, it was out of range of my diaper so it stayed together. My first night I didn't have much to treat it with. XP Medical's Absorbency Plus seem to be immune to vaseline's disassembly for the most part, lacking leak guards.
My rash didn't get any worse, but didn't improve either. I only got improvement with some hydrocortizone-based creme. That makes a poor barrier, but does promote healing. five days later and the redness is finally fading away. It took that long because I could only use vaseline during the day. In the evening and overnight it was hydrocortizone.
BTW, do not use baby lotion on a rash. The burning I had prior to trying on lotion was VERY MILD compared to the fire that was set off when I applied lotion. Could not get it off fast enough. But OTOH it gave me a very fresh perspetive of just how bad it could be hurting, so it was actually a very nice relief to get it washed off and get back to the comparatively mild feeling of the rash.
My favorite straight batter cream is Medlines Restore Remedy line, you put it on wait a minute or two to dry and diaper up, it is very clean and you would not know it I'd there if you did not apply it, they have several "versions" of it one of my favorite is a one step cleaner and barrier just take a cloth clean yourself up and a minute or two later it dries as your barrier available in both a liquid or non aerosol foam pump, it great stuff and even as the amino acid building blocks that nurture and promote healthy skin, if I am going to be wet for prolonged period and unable to change I go with straight zinc oxide , you can get a script from your Doctor for it , if you go with OTC zinc it has petroleum added to make it more spreadable and therefore lethal to your dispees and plastic pants.
As for taking on an actual rash or burn or pressure injury Calmoseptine is unmatched my Colon Rectal Doctor had me use it on hemorrhoids worked great and my G/F had a pressure injury ( aka pressure sores, skin breakdown and her physiatrist and neurologist had her use it ) prior to that I used it on occasional burns and rashes, you can get it on Amazon or if you have a local Wallgreen's pharmacy they usually have it on the shelf ( most other stores can order it, but usually don't stock it in the US ), it does have a slight fragrance or odor if you prefer , most people are unfamiliar with it, so you will not smell like a baby as with A & D or Desitin, if you want something completely odorless and that will fight odor from a wet diaper I have been getting familiar with the Balmex for adult bladder leakage , it seems to be pretty stable , odorless yet has things in it to neutralize urine and prevent odor ( it's currently under long term testing by myself ) so far pretty happy with it , you can get in Jars and also small tunes for daily carry, it is quite thick so yah gotta give it a good squeeze to get it out of the tube.