The fenus nauticum was a type of mutuum, but like most mutua it's hard to classify it as either "insurance" or as a loan. The Romans probably adopted the practice from the Athenian δάνειον ναυτικόν, or "maritime loan," of which there were two kinds, a payment for a one-way journey (which doesn't necessarily mean the ship wouldn't return back to its home port, only that the lender would not fund its return journey) and a payment for a round-trip. By the second century, B.C. at the latest the practice had become thoroughly accepted in the Roman world, as Plutarch mentions Cato the Elder loaning money for sea voyages, a practice which by Plutarch's time had still not been accepted as a reputable way of making money, common though it was. Cato, according to Plutarch, would only lend to large parties, over fifty with a number of ships, and would invest in a share of the voyage, sending a freedman of his along to make sure things were going alright--apparently the lender had a fair amount of say in the proceedings.

The fenus nauticum as it is conveyed to us by the jurists consisted essentially of a loan to a company or individual intending to transport cargo by sea. Sea travel was by far the safest and fastest means of transporting cargo in antiquity, but it was still fraught with the more or less random dangers of weather and (particularly in the Republic) piracy, and merchants were chronically underfunded, relying on loans to take on cargo after the expense of maintaining ship and crew. The lender agreed to pay out a certain amount, while the borrower promised to pay a rather high rate of interest after the sale of cargo--Justinian managed to fix the interest payments to 12%, and before that they probably could reach as much as 30%. In the event that the ship was lost the debt was forgiven--it's not quite right, therefore, to call this insurance, since the borrower didn't actually get paid anything in the event of a loss. We have an example of a Roman contract from the Digest, which lays out that the parties agree as to the home port and intended port of the journey, the period over which the loan will be considered good (200 days according to the Digest's transcript), establishment of the insured cargo as collateral, and a few other points. Of particular interest, at least for your question of fraud, is the stipulation that the lender will send along a slave to collect the interest and, presumably, make sure nothing fishy is going on. That lines up pretty well with what Plutarch tells us about Cato--moreover, Cato's case tells us that the lender could lend to entire parties, not just individual ships (and, Plutarch says, it was safer to insure in bulk), and could choose to lend only a portion of the necessary cost.

The fenus nauticum was one of many types of credit transactions possible in the Roman world, and although it was probably one of the more profitable (though risky) ones it was probably not the sole source of income for most lenders. Money-lenders generally fell under the category of negotiatores, a class of professionals that is pretty hard to describe. As the OCD puts it: "The term negotiator was rarely defined precisely, because most such money-men had investments in a whole range of property and activities." While, for example, senators could not be mercatores, merchants, and considered themselves separate from negotiatores they often held enterprises which elsewhere we find negotiatores also holding, as is the case with Cato--in particular lending. A negotiator might be involved in maritime loans, agricultural loans, land investment (probably the most common enterprise for men of wealth in Roman society, since it was so stable), general credit, and of course publican companies. Loaning and credit was a big business in the Roman world--Seneca, for example, even loaned to entire provinces! So while it's possible that a businessman might specialize only or mainly in the fenus nauticum it's somewhat unlikely--and if he did it's even more unlikely that he could become fabulously wealthy, unless he, like Cato, already possessed some sizable assets, probably in stable land investments