Every loan you take costs you 25% interest (You get 4, you pay 5) assuming you pay it back. If you don't, it costs you 75%. The "Interest" tile is an additional surcharge. Now that should be based on number of loans, but given the units in the game (and general gameplay) it doesn't ... unless you take another loan to cover it, in which case you've effectively gotten hit up another 25%.

Now, the funny thing is that taking a new loan is often a pretty good deal, it helps your cash flow. But businesses are like that, they often take out loans when not strictly necessary (or when the could structure their businesses to avoid it) for cash flow.

Regarding theme, the real question is -- how long is a round? Years? Given you start with wooden ships and how the buildings pop up, that seems reasonable but it may be wrong. If each round is several years, then the thematic arguement that interest should be charged on each loan seems reasonable. Also, what were interest charges at the time? We're used to 5-10% per annum for stable businesses. If the game only covered 5 years of the "Modern world" then 25% interest would be low, but the ineterest payments (and occassional extra loans) would bump that up. But I imagine that interest rates were much higher back then. Contrary to that, the bank in this game is reasonably confident that nobody will go bankrupt, so these loans are surely AAA rated bonds... (given that AAA bonds are going to bankrupt cities and states, these are even better. Even losing scores in Le Havre are 100+, not bad with starting with 5 florins...)

So I guess in terms of theme, per loan interest seems reasonable, but the "one time 25% + some floating interest that doesn't vary as much as it should" isn't that bothersome.