Sploshing, also known as the Wet and Messy (WAM) fetish, can involve the full-bodied clothed or naked enjoyment of liquids in a sexual way. These liquids can be edible and more pourable foodstuffs than just liquids alone. They might also be non-edible such as mud, foam, or baby oil. Sploshing can be enjoyed by yourself in a masturbatory fashion or with a partner...or even multiple partners!
You're correct in your assumption that some foods are better than others for sploshing. This isn't just because they slide and ooze in a more sensual way. It's also for practical and health reasons.
Many women suffer adverse reactions after using sexual lubricants or foods that contain sugar inside of their vagina. Both can upset the natural pH balance of the vagina and lead to irritations such as yeast infections. Other women don't suffer any negative effects. Your body remains your own responsibility. Anything that you use internally is done at your own risk. So, a small sampling before the main event to check for negative reactions is always advisable. In a similar vein, it's not a good idea to insert foods into the anal passage because they can travel up inside of the body and lead to dangerous or embarrassing situations.
You can still happily splosh without putting foods or liquids inside of your body cavities. You can simply enjoy the sensation of the foods on the outside of your body as you slip and slide, clothed or naked, alone or together with a partner.
What do you get messy with? Sweet treat fans often favor gateau style cakes, whipped cream, squirty cream, yogurt, custard, jelly, gelatin, and ice-cream. Savory sploshers can be found enjoying baked beans, raw eggs, ketchup, and various pies.
Since sploshing is, by nature, incredibly messy, you should make sure that you are prepared. A PVC sheet on the floor or over a bed should be enough to contain small samplings of food sex such as fruity insertions and a drizzle of chocolate sauce, but a vat of custard will require a bit more containment. Maybe fill a bath to share or take jugs of pourables (and drainables! Don't block your pipes!) into the shower. If you're lucky enough to have a garden which isn't part of the public view, try a paddling pool or buy an inflatable wrestling ring.
After you've enjoyed a delicious sploshing session, take care during clean up as well. Hot baths and showers will cook dairy smells into the skin. Instead, use cool to tepid water unless you want to smell like a dairy farm for about a week.
Above all, have fun. If you want to learn more about sploshing, you can read my in-depth article here.