What floats Bill’s boats

Mayor de Blasio is making a splashy election-year investment in building a city-owned, taxpayer-subsidized, $2.75-a-ride ferry fleet. Spending big on the boat network is an odd transit priority for an equity-obsessed mayor who claims to be too tapped out to defray the subway and bus costs of the poorest New Yorkers.

Ferries, in this city of rivers, generally make sense. Gotham is an archipelago with boroughs on three islands and a peninsula, and increasingly diverse economic activity near the shore.

Yet it wasn’t until entrepreneur Arthur Imperatore stepped up 30 years ago that the modern city realized private ferries could thrive as they had centuries ago.

He built NY Waterway, which led to a robust, competitive market — as of last year, there were three private operators — without any public help.

Now comes Commodore de Blasio, set to launch his navy beginning May 1.

The mayor went all in, spending a third of a billion dollars on 20 brand-new, custom-built vessels ($4 million each), and 10 new docks and, crucially, a subsidized fare far below the market rate.

Current private East River ferries charge $4 weekdays and $6 weekends. That will drop substantially when de Blasio takes over.

With $30 million in tax dollars a year for six years, Billy Boat fares will match MetroCards: $2.75 with free transfers to other boats and discounts for seniors, kids and folks with disabilities.

The public cushion is higher than it needed to be, especially since routes are based partly on customer demand and partly on political clout. Which is why the Rockaways, Bay Ridge and others will be well served, no matter their ridership.

The cost is higher still because de Blasio ruled out making out-of-towners and tourists pay more, which would have been a fine idea.

Private ferry fleets can’t fight City Hall when City Hall is driving down fares like this. No wonder both Billybey Ferry and New York Water Taxi have folded up shop and sold out.

Given that there was another way — NY Waterway and the two defunct lines offered to contract with the city for the new service, using existing staff and boats — the all-in gamble had better work. That subsidy had better not grow.

We like ferries as much as the next landlubber, but to govern is to choose.

A better use of the $355 million — one that would reach hundreds of thousands more people in much more need — would be providing halfprice MetroCards to 800,000 New Yorkers under the poverty line, which for $200 million a year could help vastly more people than any ferry lines.

At just two rides a day, half-price MetroCards would match a year’s worth of ferry trips in only three days.

De Blasio says there’s just no room in the budget for the so-called Fair Fare plan.

If true, how about offering half-priced MetroCards to the poorest of the poor, for starters? Or lessening the discount? If the goal is the greatest good for the greatest number, cheaper subway and bus rides beat artificially inexpensive ferries any day.