My Father has been renting us a house in OBX every summer since I was a kid and were both pretty affluent now and I'm floating the idea of us going in together on an Oceanfront somewhere in OBX that we can use a few weeks a year and have it pay for itself via rentals the rest of the year. 

So at end of day your invested capital is only on the float, under 12mnths.

Silly terms were 10.35% floating rate for 10 years with 35% down! 

You will see 14 days floating around. 

In this situation, I would try to use a line of credit or credit cards to float the EMD.

Pay it off with a commercial DSCR refinance loan once you've gotten the rents up to an amount that will allow you to cash out on your HELOC.I like using my HELOC but only in short-term scenarios because it's a floating rate and I can't control when or how the bank will make adjustments to the payback variables. 

If you use a HELOC, I would think about a BRRRR-ish strategy (No perfect BRRRR exists right now unless you go full off-market door knocking/postcard campaign/etc) to where you can refinance and get some of that money back, so you don't have $25k chunks of money at a 9-10% interest rate floating out there indefinitely.

Why wasn’t this idea floated out the last few years when housing costs took off?

You could do a HELOC (Floating Interest), Hybrid HELOC (Fixed at EACH time of Draw but the draw rate is based on the Interest at the time of Draw), or a 2nd Mortgage (Permanently Fixed at Origination)I prefer Hybrid HELOCs as the rates are fixed at the time of draw and that stability allows you to plan for the future. 

A house hacker with a  W-2 income can often times float some rehab costs on a duplex through cashflow or supplement some with their income.