to be honest, you simply wouldn't get away with "not investing any money in renovating or improving it". it's not like a house, which you can let fall into tatty disrepair and some mug will still buy it. everything on, in, and about your boat is functional - even the furniture to some degree as it has to be ergonomic as you live in a six foot wide corridor. the lights in my boat were naff when i got them, so i replaced them with LEDs. cheap, but makes a huge difference and makes the boat more presentable.
there are certain things you have to do anyway - you have to black the hull every 3-4 years, you have to have a boat safety certificate every 2 years (which requires an extensive examination), you need to check the plating every so often (usually through ultrasound) so that it meets legal requirements.
for the sake of argument, if you left the boat in exactly the same state you bought it (with BSS & blacking & things all up to date or as it was) and, for example, replaced any things you'd broken, there's no reason why you wouldn't be able to sell it for the same or same + inflation..... bearing in mind the state of the market. right now, as i said, the demand just keeps growing, so the prices keep upping. there's limited space and limited boats.
you CAN definitely tart up the boat - some people are obsessed with their boat looking ultra trad - but it doesn't matter how it looks, really, except to people who don't care if it works. if the boat works the same and you've taken care of the engine, no reason why it should depreciate. an older boat is more likely to have an extremely good engine. my friend's boat is a fully working 1930s workboat and it's not selling for any less than they bought it for (shameless plug? who's to say): https://www.instagram.com/57forsale/