Here's a screenshot from my latest game.http://solumslekt.org/temp/1725_stats.png
The year is 1725, and I'm already on my merry way to making a fortune. (I was kind of lucky in the start and got a couple of subsidies on routes that I was about to set up anyway, but I'm not a habitual subsidy hunter.) The transport means is the horse carriage (from the eGRVTS2) carrying 8 tons (20 km/h for 2-horse and 26 km/h for 4-horse), and the measly brig (from the Sailing Ships) which freights 50 tons at 20 km/h. Yet, this is sufficient for doing major work and making good return on investments. Part of the success is due to andythenorth's Fish2 set, which makes it profitable to run those early sailing ships. Before that, I hardly was able to make the brigs pay for their annual costs and their lifespan investment. Note that the RVs barely return a 2:1 income over running costs, while the ships return a whopping 5:1. That may seem insane, but I think that it reflects the realities rather well: If you can afford to buy ships and have got the brains to use them, you're soon swimming in gold.
You can haul almost any amount of cargo with those humble means. True, you can only transport around 100 tons a month on a dedicated road filled with 2-horse lorries. In this screenshot I've got 4-horse carriages hauling grain from three farms, each yielding 63 tons of grain. That's 127 tons a month with a 67% station rating, and the transit isn't clogged. But that's about the limit of land transport capacity in that era. Any further increase in capacity will have to be in the form of canals rather than roads.
The ships, carrying the grain to a nearby brewery, are deployed according to the Stalin method: Just pour in enough of them, and things will sort themselves out. But, over time, you should of course trim the amount of ships to the least number required to do the job adequately. Not only do ships lying waiting for cargo make no money, but the longer the wait until they get their regular depot service, the more unreliable they become and hence they will break down too often and do a lousy job. It's absolutely required to set explicit depot service orders at regular intervals for these slow vehicles; I usually send them to the depot for each round-trip, and at trips exceeding the regular service interval, I will set up depots along the route and tell them to go there.
Further, this game style has taught me to think in general of transport as a pipeline: What is stuffed into one end, will eventually come out at the other end at about the same rate.
For a given amount of cargo to reach its destination, you don't really have to think about the speed of transport at all; what matters is the amount that you feed into the pipeline, of course given the total capacity of the line as a limiting factor. For ships in OpenTTD, that kind of limit doesn't exist at all.
The speed will of course have an impact on the profit, and you'll soon learn about the burners; eg. you don't carry fruit at any distance in those circumstances. But stone, coal, clay, and stuff like that will give you good profits. Lumber and building materials will give you excellent profits over medium distances. But the sweet spot seems to be around 100-200 tiles for any kind of reasonably durable cargo. And, as andythenorth has observed, fishing is like printing money