Last Night's TV: Martin Clunes: Man to Manta/ITV1<br />The Sinking of the Laconia/BBC2<br />How Drugs Work/BBC3

On reflection, Martin Clunes might not have been the best choice to present Martin Clunes: Man to Manta. For one thing, he doesn't like diving – knows how to do it but doesn't think it's natural, and has problems breathing underwater. Also, he's not a fan of fish. "I like to cuddle and smell and stroke animals," he tells us. Manta rays, the giant ghost-like creatures he's here to study, don't particularly lend themselves to cuddles.

Still, he had time to get over that. A quick trip to Atlanta to meet Nandi, a manta who was rescued after getting trapped in nets off the coast of South Africa, at the Georgia Aquarium sorted that out. She was, observed Clunes enthusiastically, "mesmeric". So mesmeric that he found it in himself to swallow his fear of diving and climb in her tank.

The trip wasn't limited to Georgia. Somehow, despite the rather limited subject matter, Clunes managed to cover half the globe. One minute he was in America oogling Nandi, the next he was in the Cayman Islands, then Ecuador, Sri Lanka and the Maldives. Along the way, he was shown first hand the effects of hunting and climate change on mantas, sharks and turtles. Sought by fisherman for their medicinal value in the Far East, the rays tend to be killed just for one body part: their brain, for instance, or their gills, thought to be able to "filter" toxins from the body the way they filter plankton.

Arbitrary though Clunes's casting may have been, it's difficult to take issue with it. He's irrepressibly likable, burbling amiably to everyone he meets. The point was for him to swim with the mantas during feeding, though it was harder than he anticipated. Not only did he have difficulty doing the actual swimming, but the rays weren't terribly easy to locate. It took several expeditions for Clunes to get his dinner-time experience. When he did, though, it was memorable: 40 rays somersaulting and cartwheeling through clouds of plankton. And he got his cuddle, too. Not with a manta, but with a baby turtle, one of the green ones being raised by environmentalists in Sri Lanka. It even had fur. Well, a little.

Much has been made of Alan Bleasdale's The Sinking of the Laconia. For one thing, it's his first TV gig in more than a decade. For another, the plot flies in the face of much conventional wisdom. Until recently, the tale of the passenger ship, which was carrying hundreds of civilians alongside British troops and Italian prisoners of war before being sunk by torpedoes fired from a nearby German U-boat, was a straightforward one of Allies vs enemies. Last night, we saw a different side of the story, one in which the U-boat's captain, Werner Hartenstein, emerged as an unlikely hero, risking his life and military career to execute an audacious rescue operation.

Andrew Buchan starred as the upstanding Captain Thomas Mortimer (based on the real-life Thomas Buckingham) and Ken Duken as the Commander Hartenstein. Initially, the two were juxtaposed in classic jingoistic style: Mortimer, the compassionate, civilised gent, and Hartenstein, beardy and brutish, presiding over a crew of drunken womanisers, types who slaughter sharks for dinner and torment weaker crew members with adolescent initiation rituals.

Hartensen, assuming the Laconia was carrying only British troops, ordered an attack, and the great boat went down. It all got a little Titanic. Inevitably, there weren't enough lifeboats and people remained hopelessly unaware of how to evacuate. Mothers lost their babies and couples were forced to say farewell in the chilly waters of the Atlantic. Then things took a turn for the unexpected. Realising his error, Hartenstein brought his boat to the surface and commenced a rescue operation to bring them on board, offering food, medical supplies and shelter.

Bleasdale, unsurprisingly, has done a stellar job in bringing the Laconia's tale to life, combining human storylines with historical artefact to weave something truly gripping. As the secretive Hilda, a lone German among the Laconia's English upper-classes, Franka Potente (previously seen as Matt Damon's action-figure sidekick) is calmly compelling. Buchanan is just as good as the genteel Captain Mortimer. The result is a history lesson of the most enjoyable – albeit poignant – sort. We don't yet know what becomes of Hilda, Mortimer et al – the film concludes tonight - but by the close of play last night I was so involved that I can only hope it's not too terrible.

In How Drugs Work we got to see the effects – physiological, psychological – that lighting up a spliff has on the body. There's the dizziness, the light headedness, caused by lowered blood pressure. There's the loss of memory, though, curiously, an enhanced ability to connect ideas. And there's the psychosis and the paranoia. We met Imogen, a stressed-out office worker who smokes to unwind, and A-level student Mike, who still manages to leave school with a clutch of A*s. And we went to California where medicinal marijuana is legal. It was all very interesting, if a little pious. Yes, it increases your risk of a heart attack, but what about tobacco? Yes, it affects some people worse than others, but so does alcohol. Incidentally, did you know that there were more weed dispensaries in Los Angeles than there are Starbucks? Well, now you do.

a.jarvis@independent.co.uk; twitter.com/aliceazania