Isn't a gondola ride at least a little romantic?
Depends on what you want in making assumptions. Then again I could easily assume that those on the boat might be siblings, other familial combinations, or just two pals going out for fresh air. It sure would be nice if everything could only be perceived in just one way, though... well, not always, but maybe sometimes perhaps...
Some people intimate there was something between Three and Jo,
Mostly around 2007 or so, back when the sexualization of the show was at its then-peak. Thankfully you'll find some of the makers of Three's era almost looking ill in commentaries/interviews on some of the DVDs released at the time reminding of the basics of the duo. While some TV in the 1970s was bawdy and innuendo-driven ("Are You Being Served?", the "Carry On" films, and a slew of other shows and movies), not all were and definitely not children's shows or family shows after a point. 3/Jo were definitely more teacher/student or father/daughter at most.
and I've even seen people comment that there's a hint of something between Two and Jamie, but on the whole you're right, there's pretty much nada!
Diamanda Hagan is one (YouTube channel) claiming the actors based their banter as if they were a "couple". The two actors clearly had much onscreen chemistry, with character traits complementing one another.
There were sparks between the First Doctor and Cameca in The Aztecs.
Only when the Doctor decides to leave the TARDIS to pick up the broach he'd placed back in the tomb, which was given to him by Cameca. Can still be inferred as friendship or emotional bond, and given the Doctor's reaction to pandering to Cameca to get tomb plans only to find out the cocoa bean preparation was a marriage ritual and he promptly spazzes at the camera... so, nope, no character was horny during the events of that episode. Except for the Perfect Victim handpicked for Susan by Tlotoxyl there... Trolltoxyl?

But what was said and what was left to the imagination made it even more horrific, something 70s Who and onward forgot about.